A Collection of German Science Interests in the

Next Generation Very Large Array

M. Kadler1, D. A. Riechers2, A.-K. Baczko3,20, H. Beuther4, F. Bigiel5, T.
Birnstiel6, B. Boccardi3, L. Boogaard7, S. Britzen3, M. Brüggen8, A.
Brunthaler3, P. Caselli9, D. Elsässer10, S. von Fellenberg3, M. Flock7, C. M.
Fromm1, M. Hoeft11, R. P. Keenan7, Y. Kovalev3, K. Kreckel12, J. Livingston3,
A. P. Lobanov3, H. Müller3, E. Ros3, P. Schilke2, L. Spitler3, T. Ueda4, E.
Vardoulaki11, S. Vegetti13, C. Wendel1, M. H. Xu14, G.-Y. Zhao3, A. Basu11, J.
Becker Tjus15, S. Bernhart16, E. Bonnassieux1, G. DiGenaro7, F. Eppel1, H.
Hase16, D. Hoang7, M. Kaasinen17, E. Kun15,21,22, Y. Lin9, K. Mannheim1, K.
Menten3, D. M. Powell13, L. Rezzolla18, L. Ricci1, E. Schinnerer7, K.
Schmidt10, N. Sulzenauer3, A. Tursunov3, F. Walter7, A. Weiß3, G. Witzel3, S.
Wolf19, J. A. Zensus3

1JMU, Würzburg, 2Univ. zu Köln, 3MPIfR, Bonn, 4MPIA, Heidelberg, 5AIfA, Univ.
Bonn, 6LMU, Munich, 7MPIA, Heidelberg, 8Univ. Hamburg, 9MPE, Garching, 10TU
Dortmund, 11TLS, Tautenburg, 12Univ. Heidelberg, 13MPA, Garching, 14GFZ,
Potsdam, 15RUB, Bochum, 16BKG, Frankfurt, 17ESO, Garching, 18Goethe Univ.
Frankfurt, 19Univ. Kiel

With international contributions by

A. Mus23, V. Toth24, A. Alberdi25, M. Benisty26, P. Cox27, J. C. Guirado23, M.
D. Johnson28, M. Juvela29, M. Neeleman30, I.N. Pashchenko31,32, M. Á. Pérez
Torres25, K. Perraut33, M. Zajaček34

20Chalmers Univ. of Technology, Gothenburg, Sweden, 21Konkoly Obs., Hungary,
22CSFK, Hungary, 23Univ. de València, 24Eötvös Univ. Budapest, 25Inst. de
Astrofísica de Andalucía, Spain, 26IPAG, France, 27IAP, France, 28CfA Harvard
& Smithsonian, USA 29Univ. Helsinki, 30National Radio Astronomy Observatory,
USA 31Lebedev Physical Institute, Russia, 32Moscow Inst. of Physics and
Technology, Russia, 33Univ. Grenoble Alpes, France, 34Masaryk Univ., Czech
Republic

###### Contents

  1. 1 A Foreword by the Curators of this Community Paper
  2. 2 Science Cases
    1. 2.1 AGN jets at all scales
    2. 2.2 Small-scale structure of the warm and cold neutral medium (WNM & CNM) with HI emission and HI self-absorption (HISA)
    3. 2.3 HI Studies of Nearby Galaxies
    4. 2.4 Imaging Young Solar System Analogues in Formation
    5. 2.5 The acceleration and collimation zone in AGN jets
    6. 2.6 Redshift Measurements
    7. 2.7 Kinematics of cold gas in high-redshift galaxies
    8. 2.8 Detecting Supermassive Binary Black Holes with the ngVLA
    9. 2.9 Radio studies of the cosmic web
    10. 2.10 Galactic Structure
    11. 2.11 Chemical and physical conditions at the dawn of star and planet formation
    12. 2.12 ngVLA studies of gravitational-wave triggers and other energetic transient events
    13. 2.13 Hyper Compact Radio sources in the Galactic Center
    14. 2.14 Substructures of dust rings in protoplanetary disks
    15. 2.15 Probing SgrA⋆ flares with the ngVLA
    16. 2.16 Jet Launching and Radiation Microphysics
    17. 2.17 Galaxy clusters with ngVLA
    18. 2.18 Resolving the Doppler Crisis
    19. 2.19 VLBI Probes of Neutrino Emission in Blazars
    20. 2.20 Low-$J$ CO Line Excitation
    21. 2.21 Radio-optical link: AGN-based VLBI and Gaia fundamental reference systems and their application for astrometry and astrophysics
    22. 2.22 Blazars as neutrino candidates: complete sample studies
    23. 2.23 Dense gas in nearby galaxies
    24. 2.24 Wide-band polarization AGN studies, covering scales from parsecs to kiloparsecs, measuring extreme Faraday rotation and complex Faraday effects
    25. 2.25 Astrophysical searches for physics beyond the Standard Model
    26. 2.26 Physics and astrometry with photon rings
    27. 2.27 Scattering mitigation of the Galactic Center using ngVLA
    28. 2.28 The Cosmic Density and Excitation of Cold Molecular Gas
    29. 2.29 Molecular Line Absorption Against the CMB as a Probe of Cosmological Parameters
    30. 2.30 VLBI probes of supernovae, supernova factories and starburst galaxies
    31. 2.31 Zooming into Feedback Engines
    32. 2.32 Studies of fast radio Bursts above 2 GHz with the ngVLA
    33. 2.33 Study of the Dense ISM with the ngVLA
    34. 2.34 Star and Planet Formation with ngVLA
    35. 2.35 Planet Formation at the Innermost Region of Protoplanetary Disks
    36. 2.36 From near-infrared to centimeter: comprehensive understanding of the inner planet-forming region
    37. 2.37 Big Data, surveys, classification
    38. 2.38 Constraining the nature of dark matter with strong gravitational lensing
    39. 2.39 Sub-Structures in Blazar Jets and Synergetic TeV Observations
    40. 2.40 Antenna positions with a sub-mm repeatability
    41. 2.41 High-precision $\lambda$-astrometry at mm-wavelengths with the ngVLA
  3. 3 Bibliography

##  1 A Foreword by the Curators of this Community Paper

Matthias Kadler (JMU Würzburg) & Dominik Riechers (Univ. Köln)

The Next Generation Very Large Array (ngVLA) project (Fig. 1),111Reference
design: https://ngvla.nrao.edu/page/projdoc developed by the international
astronomy community under the lead of the National Radio Astronomy Observatory
(NRAO; [266]), has recently been identified as one of the top-priority new
ground-based observatories in the 2020 US decadal report "Pathways to
Discovery in Astronomy and Astrophysics for the
2020s"222https://nap.nationalacademies.org/resource/26141/interactive/. As
currently designed, it is expected to be a transformative scientific
instrument and a key facility for the US astronomical community to be
constructed by the next decade. In terms of scientific capabilities, the ngVLA
will be a key complement to facilities targeting lower radio frequencies such
as the Square Kilometre Array (SKA). Presently, the project is looking to
provide opportunities for international cooperation. Given the long-standing
tradition of significant usage of NRAO facilities by the German community (see
Fig. 2), Germany would be a natural partner, but any such potential engagement
requires further exploration.

In response to this opportunity, an exploratory
workshop333https://events.mpifr-bonn.mpg.de/indico/event/306/ for members of
the German community interested in the ngVLA project was held at the Max-
Planck-Institute for Radio Astronomy (MPIfR) on 14 December 2022 in Bonn. This
workshop was met with great interest and has brought forward a number of
interesting ideas. A second, equally stimulating
workshop444https://events.mpifr-bonn.mpg.de/indico/event/339/ was held at the
Max-Planck-Institute for Mathematics in the Sciences in Leipzig on 27-28
September 2023.

Given the considerable participation by the German community in both
workshops, it was decided to collect the science interests of this community
through short contributions. These contributions are collected in this volume.
The main purpose of this volume is to demonstrate the breadth of the current
interest across the German community in the facility as a basis for further
discussion. This document is supposed to be a ‘living document’, i.e. it can
and should be updated with additional science cases in the
future.555Contributors are asked to use this template:
https://www.overleaf.com/project/651bde92dbe7c6e9e8308574 Interested authors
should contact us if additional science topics should be included that are not
represented in this volume. Also, this document is not meant to be a "Science
Book". It is rather a consolidation of goals and ideas. Among the science
cases in the existing ngVLA Science Book, 18 already involved German
researchers, a subset of which are also represented in this document. We
encourage authors to expand any new ideas from this document into full science
cases for the ngVLA Science Book, in collaboration with our US colleagues.
E.g., it was noted that the German community has a particular interest in
very-long-baseline science cases that are currently not captured prominently
in the ngVLA Science Book.

The current timeline for the ngVLA, as presented at the 2023 workshop by NRAO,
foresees delivery of the first prototype antenna, built by the Germany-based
company mtex antenna technology, at the testing site in New Mexico in 2024.
Construction of the array is anticipated to start in 2027/28, with Early
Science opportunities in the early 2030s, and completion of construction and
Full Science Operations expected around 2037.

Figure 1: Artist’s conception of the mtex design for the ngVLA prototype
antenna (credit: Sophia Dagnello, NRAO/AUI/NSF). The array will consist of
three components: a main interferometric array with 214 $\times$ 18 m
reflector antennas covering baselines up to 1068 km, a short baseline array
with 19 $\times$ 6 m antennas covering the shortest baselines, and a long
baseline array with 30 $\times$ 18 m antennas covering baselines up to 8860
km. It will be deployed in the southern US and northern Mexico, with long
baselines reaching up to Canada and down to Puerto Rico and the US Virgin
Islands. This will be sufficient to achieve sub-milliarcsecond resolution. The
array will operate between wavelengths of 21 cm and 3 mm. Figure 2:
International usage of NRAO facilities for the years 2011–2021, kindly
provided by NRAO (figure sourced from a presentation given by Tony Beasley at
the 2023 workshop). Astronomers located in Germany are the most prolific users
outside the US through the "open skies" agreement, demonstrating the major
interest in these facilities in the German community. The ngVLA will be the
natural evolution of both the Karl G. Jansky Very Large Array (VLA) and the
Very Long Baseline Array (VLBA), and thus, is of major interest to astronomers
located in Germany.

##  2 Science Cases

Topical Areas of Science Cases

1: Stellar Astrophysics, 2: Solar System, Planetary Systems and Habitability,
3: Circuit of Cosmic Matter (incl. star formation), 4: The Galaxy and the
Local Group, 5: Galaxies and AGN, 6: Cosmology, Large Scale Structure and
Early Universe, 7: Extreme conditions in the cosmos, fundamental astrophysics

Principal |  Science Case | Page | Topical Area
---|---|---|---
Author |  |  | 1 | 2 | 3 | 4 | 5 | 6 | 7
Baczko |  AGN jets at all scales | 2.1 |  |  | ✓ |  |  |  |
Beuther |  Small-scale structure of the warm and cold neutral medium (WNM & CNM) with HI emission and HI self-absorption (HISA) | 2.2 |  |  | ✓ | ✓ | ✓ |  |
Bigiel |  HI Studies of Nearby Galaxies | 2.3 |  |  | ✓ | ✓ | ✓ |  |
Birnstiel |  Imaging Young Solar System Analogues in Formation | 2.4 |  | ✓ |  |  |  |  |
Boccardi |  The acceleration and collimation zone in AGN jets | 2.5 |  |  |  |  | ✓ |  |
Boogaard |  Redshift Measurements | 2.6 |  |  |  |  | ✓ | ✓ |
Boogaard |  Kinematics of cold gas in high-redshift galaxies | 2.7 |  |  | ✓ |  | ✓ | ✓ |
Britzen |  Detecting Supermassive Binary Black Holes with the ngVLA | 2.8 |  |  |  |  | ✓ | ✓ | ✓
Brüggen |  Radio studies of the cosmic web | 2.9 |  |  |  |  |  | ✓ |
Brunthaler |  Galactic Structure | 2.10 |  |  |  | ✓ |  |  |
Caselli |  Chemical and physical conditions at the dawn of star and planet formation | 2.11 |  | ✓ |  |  |  |  |
Elsässer |  ngVLA studies of gravitational-wave triggers and other energetic transient events | 2.12 |  |  |  |  | ✓ |  | ✓
von Fellenberg |  Hyper Compact Radio sources in the Galactic Center | 2.13 | ✓ |  |  | ✓ |  |  | ✓
Flock |  Substructures of dust rings in protoplanetary disks | 2.14 |  | ✓ |  |  |  |  |
Fromm |  Probing SgrA⋆ flares with the ngVLA | 2.15 |  |  |  | ✓ |  |  | ✓
Fromm |  Jet Launching and Radiation Microphysics | 2.16 |  |  |  |  | ✓ |  | ✓
Hoeft |  Galaxy clusters with ngVLA | 2.17 |  |  |  |  | ✓ | ✓ |
Kadler |  Resolving the Doppler Crisis | 2.18 |  |  |  |  | ✓ |  | ✓
Kadler |  VLBI Probes of Neutrino Emission Processes in Blazars | 2.19 |  |  |  |  | ✓ |  | ✓
Keenan |  Low-$J$ CO Line Excitation | 2.20 |  |  | ✓ |  | ✓ |  |
Kovalev |  Radio-optical link: AGN-based VLBI and Gaia fundamental reference systems and their application for astrometry and astrophysics | 2.21 |  |  |  |  | ✓ |  |
Kovalev |  Blazars as neutrino candidates: complete sample studies | 2.22 |  |  |  |  | ✓ |  | ✓

to be continued on next page

Principal |  Science Case | Page | Topical Area
---|---|---|---
Author |  |  | 1 | 2 | 3 | 4 | 5 | 6 | 7
Kreckel |  Dense gas in nearby galaxies | 2.23 |  |  | ✓ |  | ✓ |  |
Livingston |  Wide-band polarization AGN studies, covering scales from parsecs to kiloparsecs, measuring extreme Faraday rotation and complex Faraday effects | 2.24 |  |  |  |  | ✓ |  | ✓
Lobanov |  Astrophysical searches for physics beyond the Standard Model | 2.25 |  |  |  |  |  |  | ✓
Lobanov |  Physics and astrometry with photon rings | 2.26 |  |  |  |  |  |  | ✓
Müller & Mus |  Scattering mitigation of the Galactic Center using ngVLA | 2.27 |  |  |  | ✓ | ✓ |  | ✓
Riechers |  The Cosmic Density and Excitation of Cold Molecular Gas | 2.28 |  |  |  |  | ✓ | ✓ |
Riechers |  Molecular Line Absorption Against the CMB as a Probe of Cosmological Parameters | 2.29 |  |  |  |  |  | ✓ | ✓
Ros |  VLBI probes of supernovae, supernova factories and Starburst galaxies | 2.30 | ✓ |  |  |  |  |  |
Schilke |  Zooming into Feedback Engines | 2.31 |  |  | ✓ |  |  |  |
Spitler |  Studies of fast radio bursts with the ngVL | 2.32 |  |  |  |  |  |  | ✓
Toth |  Study of the Dense ISM with the ngVLA | 2.33 |  |  | ✓ | ✓ |  |  |
Ueda |  Star and Planet Formation with ngVLA | 2.34 |  | ✓ | ✓ |  |  |  |
Ueda |  Planet Formation at the Innermost Region of Protoplanetary Disks | 2.35 |  | ✓ |  |  |  |  |
Ueda |  From near-infrared to centimeter: comprehensive understanding of the inner planet-forming region | 2.36 |  | ✓ |  |  |  |  |
Vardoulaki |  Big Data, surveys, classification | 2.37 |  |  |  |  | ✓ |  |
Vegetti |  Constraining the nature of dark matter with strong gravitational lensing | 2.38 |  |  |  |  |  | ✓ | ✓
Wendel |  Sub-Structures in Blazar Jets and Synergetic TeV Observations | 2.39 |  |  |  |  | ✓ |  | ✓
Xu |  Antenna positions with a sub-mm repeatability | 2.40 |  |  |  |  | ✓ |  |
Zhao |  High-precision $\lambda$-astrometry at mm-wavelengths with the ngVLA | 2.41 |  |  |  |  | ✓ |  |

###  2.1 AGN jets at all scales

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Anne-Kathrin Baczko Institution: Chalmers University of Technology,
Gothenburg Sweden; MPIfR Bonn, Germany Email<EMAIL_ADDRESS>
Co-authors:

Eleni Vardoulaki, Thüringer Landessternwarte Tautenburg TLS, Etienne
Bonnassieux, JMU Würzburg; LESIA, Observatoire de Paris, France; INAF, Italy

The unified model for active galactic nuclei (AGN) [379] (see Fig. 3),
specifically the physical description of the formation and evolution of
extragalactic jets, remains incomplete. One reason being, our current
understanding of jet evolution and the acceleration and collimation zone (ACZ)
up to pc scales is strongly biased towards bright, easily-detectable blazars
[compare e.g. 207]. Overcoming this limitation will require at the same time
observational constraints from the formation to the energy dissipation of
jets, including the fainter AGN jet populations, and predictions from
theoretical models. Not only will such constraints need to cover as much of
the emitting spectrum as possible, but also study as diverse a population of
AGN as possible at as many physically-relevant scales as possible.

The ngVLA [347] has the potential to provide the necessary observational
constraints, and unlock a new era in our understanding of the physics of AGN
jets and their interactions with both the host galaxy and the multi-scale
environment. This is due to three factors: (1) an unprecedented dynamic range
for continuum and spectral line observations, allowing to probe the complete
populations of jets, and thereby enabling to study the AGN/SFG dichotomy like
never before; (2) the homogeneity of the ngVLA array antenna design at
multiple frequencies (1–116 GHz), crucial for morphological and continuum
spectral studies over several scales; (3) sensitive polarization observations,
enabling us to trace the strength and distribution of the jets magnetic field,
relevant for the physical modeling of jet formation and acceleration.

Present Very Long Baseline Interferometry (VLBI) arrays can reach mas
resolution at GHz frequencies, but have a very limited field of view (FoV)
[compare e.g. 228], providing valuable constraints on the gravitational and
magneto-hydrodynamic processes describing the ACZ ([32, 33]). Non-VLBI arrays
such as ALMA or VLA, on the other hand, provide a large FoV but at the cost of
limited resolution, providing constraints on the jet propagation and growing
instabilities [373] towards energy dissipation in the outer lobes. ALMA, the
high-frequency VLA and SKA pathfinders such as the ILT have attempted to
achieve sub-arcsec resolutions with multiple-degree FoVs [261]. These operate
at very different frequency regimes, however, and therefore trace different
regions and physics of AGN jets, connecting between these regimes is
impossible with current instruments. However, this is crucial when, e.g.,
studying jet expansion as shown by [35].

The ngVLA will be key in solving this issue. The sensitivity to emission up to
arcsec scales may provide simultaneous insight into the regions not only of
formation and acceleration (sub pc–10pc scales) or of expansion (kpc scales),
but also into the energy dissipation region (Mpc jet, which the shortest
baselines of the ngVLA are sensitive to up to band 4). For the first time this
will enable an inclusive physical description of the transition between
relativistic/ballistic physics and energy deposition into surrounding media.

Summarizing, the ngVLA will enable us to access a much larger population of
AGN jets at all scales than currently available. In so doing, the current
blazar-dominated populations serving as the basis of our present understanding
of AGN jets will be balanced by healthy populations of non-blazar sources,
building a solid foundation for a complete description of jets physics in AGN.

Figure 3: Sketch of an AGN at large (left) and small (right) scales. The
region of formation, acceleration and collimation of the jets as well as
properties of absorbing structures as the torus can be studied with VLBI,
whereas the large scale instabilities and interactions with the surrounding
media, e.g. shocks along the jet or radio lobes, can currently be studied only
with telescopes as ILT, VLA or ALMA. The ngVLA will for the first time allow
to bridge the gap between both scales.

###  2.2 Small-scale structure of the warm and cold neutral medium (WNM & CNM)
with HI emission and HI self-absorption (HISA)

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ ✓ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Henrik Beuther Institution: Max Planck Institute for Astronomy Email:
<EMAIL_ADDRESS>
The structure and physical properties of the atomic gas are key parameters of
the interstellar medium. The transitions from the warm neutral medium (WNM) to
the cold neutral medium (CNM) and then to molecular gas are essential steps
for the formation of stars as well as shaping galaxy structures as a whole.
Over the last decades, the VLA has been one of the major contributors to the
understanding of HI properties in the Milky Way and external galaxies (e.g.,
[360, 395, 26]). While the classical 21 cm emission line typically probes a
mixture of the WNM and CNM, specially for Milky Way clouds, one can isolate
the CNM by studying HI self-absorption (HISA) (e.g., [127, 399, 367], see Fig.
4 for an example). Such kind of studies allowed us to characterize the
physical properties of the different phases of the atomic gas on Milky Way
scales.

Nevertheless, the studies so far suffered from severe limitations. These are
largely related to insufficient spatial and spectral resolution achievable
with current observatories and instrumentation. The HI/OH/Recombination line
survey of the Milky Way (THOR) conducted with the VLA is the best HI survey of
our Milky Way so far [26]. However, based on brightness sensitivity
limitations, the final HI data have only a spatial resolution of
$40^{\prime\prime}$ (although the VLA C-array data in principle would allow
higher spatial resolution) [398]. Furthermore, the spectral resolution of the
final datacubes is also comparably coarse around $\sim$1.5 km s-1 [398].
Hence, given observational limitations omitted in-depth studies of the small-
scale spatial structure as well as detailed kinematic analysis of the densest
HI structures that are most closely related to molecular cloud and star
formation processes.

The ngVLA will boost the sensitivity for HI observations and by that allow us
deep insights into the small-scale spatial and kinematic properties of the
atomic gas during its conversion processes into the molecular phase. With the
vastly increased collecting area of the ngVLA, the achievable brightness
sensitivity will allow us to create maps of the WNM and CNM at spatial scales
below $10^{\prime\prime}$ that correspond at typical Galactic distances of 4
kpc to linear resolution elements $\leq$0.2 pc. Furthermore, while for a few
nearby clouds narrow HISA has been detected (so-called HINSA [226]), such
studies were not possible on Milky Way scales so far. Measured line-width of
HINSA in these nearby clouds are typically below 1 km s-1, significantly
narrower than the spectral resolution of existing Milky Way-scale surveys
(e.g., THOR [398]). Hence, again the vast sensitivity improvements of the
ngVLA with allow HI studies at $\sim$0.1 km s-1 spectral resolution,
disentangling the velocity structures within atomic and molecular clouds.

To summarize, the ngVLA will allow us for the first time to investigate on
Milky Way scales the small scale spatial and kinematic atomic gas properties
that lead to molecular clouds and ultimately to the formation of stars.

Figure 4: HISA peak velocities are presented in color-scale for one large
cloud in the Milky Way (presented in Galactic longitude and latitude [367]).
The contours show the corresponding integrated 13CO(1–0) emission from the
Galactic ring survey [171].

###  2.3 HI Studies of Nearby Galaxies

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ ✓ Circuit of Cosmic Matter (incl.
star formation) $\square$ ✓ The Galaxy and the Local Group $\square$ ✓
Galaxies and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Frank Bigiel Institution: AIfA, Univ. Bonn Email<EMAIL_ADDRESS>bonn.de

Co-authors: Fabian Walter (MPIA)

Studies of HI emission in and across galaxies address a wide range of science
questions: HI emission probes the galactic potential out to very large radii
which, via mass modelling, constrains the density structure of the dark matter
halo [e.g. 78], extended HI responds to gravitational perturbations from
galaxy-galaxy interactions [e.g. 49], kinematics constrain non-circular
motions linked to gas in- and outflows in the plane of the disk [e.g. 100],
which in combination with extraplanar gas clouds provides important
constraints on present day gas accretion fueling star formation. The superior
resolution and sensitivity of the ngVLA will shed light on the nature and
physical properties of atomic gas clouds and the overall diffuse, extended HI
disks of galaxies at column densities $\ll 10^{18}\,\textrm{cm}^{-1}$, where a
break in the exponential gas profiles is expected [e.g. 48]. Such sensitive
observations will also offer key points of comparison to cosmological
simulations [e.g. 349], regarding gas accretion rates in the present day
universe, the "missing dwarfs" problem by providing a sensitive census of
"dark" dwarf galaxies and, given the exquisite spatial resolution, in
particular regarding the dark matter profile in galaxy centers ("cusp vs.
core").

Atomic hydrogen is the raw material for star formation, but how exactly the
conversion to H2 is regulated is not settled on kpc-scales [e.g. 109] and
essentially (except for the Magellanic Clouds) unprobed on the relevant cloud
scales of a few 10 pc (corresponding to around 1$"$) across nearby galaxies.
Measuring the coupling between dynamics (line widths), angular momentum around
molecular clouds, comparing them to those of the CO tracing the molecular gas,
as well as probing HI columns, power spectra and structure functions on
molecular cloud scale will all represent major new avenues. How all of these
properties are ultimately regulated by ISM physical conditions (radiation
field, metallicity) and galactic properties and how they impact the conversion
of HI to H2 are further central topics. While these measurements all exist for
the CO at such scales [e.g. 225], the excellent surface brightness sensitivity
of the ngVLA will enable matched HI observations for a diverse and significant
sample of local galaxies by pushing such studies beyond the local group. In
addition, the superior sensitivity will also allow to probe alternative
molecular gas tracers such as OH over significant areas of local galaxies.
Arcsecond-scale 21 cm mapping with the ngVLA will also probe scales where HI
absorption features against bright sources can be isolated, giving access to
HI spin temperature measurements so far only feasible at decent signal-to-
noise in the local group [e.g. 47].

Overall, while current facilities (in particular MeerKAT and the VLA) do reach
required sensitivities and resolution, mapping any reasonable area outside the
local group would still require an enormous or even prohibitive amount of
observing time (cf. the ongoing 2000 hr LGLBS survey at the VLA). On top of
that, this is currently an "either / or" situation; either mapping at high
sensitivity but approaching arcminute resolution or pushing to arcsecond-scale
which means basically giving up mapping and/or pushing to required column
density sensitivities. The ngVLA will be an indispensable tool for sensitive,
molecular cloud- / arcsecond-scale HI mapping of prominent northern sky local
galaxies with significant complementary data coverage at matched resolution
from the UV into the mid-IR from JWST.

Figure 5: VLA HI observations of the local spiral galaxy M83 from Eibensteiner
et al. [100, PI: F. Bigiel], illustrating several of the ngVLA HI science
cases: extended HI as a probe of the galactic potential, (kinematic) search
for gas accretion and radial transport as well as star formation in different
galactic environments. Pushing such observations to arcsecond, molecular cloud
scale and recovering the low-column density gas filling by far most of the
area at large radii is new observational phase space the ngVLA will unlock.

###  2.4 Imaging Young Solar System Analogues in Formation

Thematic Areas: $\square$ Stellar Astrophysics $\square$ ✓ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Til Birnstiel Institution: LMU Munich Email<EMAIL_ADDRESS>
Co-authors: Takahiro Ueda, Max Planck Institute for Astronomy, Mario Flock,
Max Planck Institute for Astronomy, Sebastian Wolf, Kiel University

Planets can form quickly in substructures in protoplanetary disks (see [219],
and Figure 6 which depicts a simulated ngVLA observation of a gas+dust
hydrodynamic simulation of a young solar system analogue). At the same time,
substructures are also considered signposts of planets [374, 292]. This pushes
the chicken-or-egg question of how planets form, and whether they are the
cause or consequence of substructure towards the earliest stages of disk
evolution. However, young disks are dense and small and therefore highly
optically thick. Observing solar system analogues requires long wavelength
observations (several mm to cm) at high angular resolution (1 au at 140 pc
corresponds to $\sim$7 mas). The ngVLA is the only instrument that can provide
this combination of sensitivity, angular resolution, and wavelength coverage.

Observing these disks and understanding the processes governing planet
formation within them are essential to study the fundamental mechanisms that
shaped our own solar system and the diversity of exoplanetary systems. The
exceptional spatial resolution of the ngVLA will be a key to understand the
complex structures of young disks. These disks exhibit intricate sub-
structures, including rings, gaps, and spirals, that are indicative of planet
formation processes.

An important feature of planet formation on scales of only few au are the
timescales: over periods of few years, possibly entire orbits of the planets
can be observed which will push radio continuum observations into the time
domain. This will enable studies of the dust dynamics and its collisional
evolution at unprecedented detail. ngVLA’s multi-wavelength capabilities,
spanning wavelength from millimeters to a decimeter, will enable a
comprehensive view of dust properties via the spatial distribution of the
spectral index (see Figure 6). These studies will strongly benefit if
polarization measurements will be available as well.

In summary, ngVLA’s unique combination of sensitivity, spatial resolution, and
wavelength coverage will make it an indispensable tool in the study of planet
formation in young disks. It will enable time domain studies, will resolve
azimuthal substructures, probe previously optically thick regions, and will
push towards the realms of terrestrial planet formation near the snow line.
These advancements will revolutionize our understanding of the formation and
diversity of planetary systems.

Figure 6: The ngVLA will be able to resolve structures in young disks at high
angular resolution. The image shows a simulated observation at 3 mm with the
ngVLA (6 mas beam, 0.43 $\mu$Jy noise level, following [318]). The disk is
resolved at around 1 au resolution, revealing the complex sub-structures and
vortices within the disk. The disk and planet setup is based on sequential
planet formation models of Lau, Birnstiel, et al. (submitted). The figure on
the right depicts the dust spectral index of the same simulation.

###  2.5 The acceleration and collimation zone in AGN jets

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Bia Boccardi Institution: MPIfR, Bonn Email<EMAIL_ADDRESS>
Co-authors:
Anne Baczko, Chalmers University Gothenburg, Matthias Kadler, JMU Würzburg,
Yuri Kovalev, MPIfR Bonn, Luca Ricci, JMU Würzburg

Very-Long-Baseline-Interferometry (VLBI) observations have been able to access
the jet launching region in active galactic nuclei (AGN), zooming in the
immediate proximity of the black hole [103]. In several nearby sources, these
have revealed that jets undergo acceleration and collimation on scales of
$10^{3}-10^{7}$ Schwarzschild radii [207, 35], finally attaining bulk Lorentz
factors of the order of tens and opening angles of few degrees on parsec
scales [307]. The physical mechanisms driving these processes are still a
matter of debate. The co-spatiality of the acceleration and collimation zone
(ACZ) observed in a handful of objects [see the case of NGC 315 in Fig. 1 -
left, reported by 317] is in agreement with theoretical predictions for mainly
magnetically-driven jets [202]. Concerning the jet confinement, however, it is
unclear whether the ambient medium also plays an important role, or if self-
collimation due to the magnetic field permeating the jet is sufficient. The
former hypothesis was first suggested by [14] based on their study of M87,
showing an approximate coincidence between the parabolic jet region and the
Bondi radius. This coincidence is however not always observed [317, 412].
Moreover, the very nature of the ambient medium in which the jet initially
propagates is uncertain. For instance, winds produced by the accretion disk
could contribute to the jet confinement as well [130]. The results collected
so far do hint at a relation between the AGN accretion mode (radiatively
efficient vs. inefficient) and the properties of the ACZ, which tends to be
more extended in sources powered by radiatively efficient, thin disks [35,
Fig. 1 - right]. However, the number of objects where the ACZ could be imaged
is still too low to make any solid statement in this regard.

The ngVLA will be the prime observing facility in this research area. Indeed,
while providing maximum angular resolution similar to that of current global
mm-VLBI arrays (NGVLA Memo 105)’, it will be characterized by orders-of-
magnitude improvements in sensitivity. This implies that the number of sources
suitable for such studies will dramatically rise, extending to the vast and
unexplored population of faint radio galaxies where the ACZ is expected to be
well resolved [e.g., 309]. The increased image fidelity and observing cadence
enabled by the ngVLA will strongly facilitate the determination of the true
jet shape in stacked images, as well as the investigation of the jet
acceleration based on kinematic studies and jet-to-counter jet ratio profiles.
Finally, the unprecedented capability of combining (sub)mas-scale resolution
with sensitivity to thermal emission will give us a complete view of the
outflow stratification properties, probing the presence and properties of
accretion disk winds and other material in the nuclear environment. The
improved line sensitivity and broader spectral line window coverage of the
ngVLA will also be fundamental for tracing the molecular jet environment down
to mm-wavelengths.

Figure 7: Left: Intrinsic speed profile derived for NGC315 based on the jet-
to-counter jet ratio in different VLBI images. The vertical line marks the end
of the collimation region [317]. Right: ACZ extent in sources classified as
high-excitation (HEG) or low-excitation (LEG) galaxies [35]. The former are
powered by radiatively efficient standard disks, the latter by radiatively
inefficient hot accretion flows.

###  2.6 Redshift Measurements

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ ✓ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Leindert A. Boogaard Institution: Max-Planck-Institut für Astronomie
(MPIA), Heidelberg Email<EMAIL_ADDRESS>
Co-authors: Dominik Riechers (Universität zu Köln), Fabian Walter (MPIA)

The (sub)mm/radio regime provides unique means to measure spectroscopic
redshifts from molecular, atomic, and ionised gas (fine-structure) emission
lines for galaxies throughout cosmic time, into regimes where UV/optical/near-
IR facilities cannot. Spectral scan surveys, both in the context of molecular
deep fields [132] as well as targeted efforts [316, 41] have demonstrated this
out to the highest-redshift galaxies [149], and have the potential to provide
a wealth of lines for high-redshift galaxies [320]. Key for efficient redshift
measurements through spectral scans are broad instantaneous bandwidths
($\Delta\nu/\nu$) and a large field-of-view, coupled with high sensitivity.
The ngVLA baseline receiver configuration from 1.2-116 GHz features large
instantaneous bandwidths that encompass the full bandwidth of Bands 1-5 from
1.2 up to 50 GHz $(\Delta\nu/\nu=1)$ and 20 GHz in Band 6 at 70–116 GHz
($\Delta\nu/\nu\sim 0.43$), covering a broad range of spectral features out to
cosmic dawn (Fig. 9). Coupled with a significant increase in sensitivity
compared to current interferometers and a larger field-of-view at low
frequencies than ALMA can provide, these capabilities open a wealth of
opportunities: Molecular deep fields and CO excitation studies are explored in
further detail in other the chapters in this volume. In addition, the [CI]
lines may prove an relevant alternative to measure cold gas masses in low
metallicity (CO-dark) systems [36, 162] with the ngVLA, in particular at
higher redshift [74, 38]. The order-of-magnitude increase in sensitivity
compared to ALMA implies that a dense gas tracers can be detected on the same
timescale as CO with current facilities [81], implying rich spectra can be
obtained for gas-rich galaxies out to high redshift (Fig. 9). These topics
highlight the broad range of opportunities for redshift measurements and
spectral line studies enabled by the ngVLA.

Figure 8: Left: Frequency vs. redshift for the ngVLA Bands 1–6 (shown in
gray), highlighting the coverage of the strongest lines (CO, [CI] and fine
structure lines) up to $z=20$. Also shown is the (planned) coverage of ALMA
(cyan) and SKA (yellow). Right: Broad-band spectrum of the dusty starburst
galaxy HFLS3 at redshift 6.3, showing a wealth of features from CO, [CI], H2O
and fainter species, including H2O absorption against the CMB at 75.9 GHz,
obtained with a NOEMA spectral scan spanning 45 GHz at 3 mm (ngVLA Band 6)
[322].

###  2.7 Kinematics of cold gas in high-redshift galaxies

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ ✓ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ ✓ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Leindert A. Boogaard Institution: Max-Planck-Institut für Astronomie
(MPIA), Heidelberg Email<EMAIL_ADDRESS>
Co-authors: Marcel Neeleman (NRAO), Fabian Walter (MPIA), Melanie Kaasinen
(ESO), Dominik Riechers (Univ. zu Köln)

The dynamical properties of distant galaxies provide key insights into their
structure and formation mechanisms. The kinematics of the cold gas can give
direct insight into, for example, the presence of disks, accretion and
mergers, the dynamical support (rotation, random motions, turbulence), and the
dynamical substructure of the galaxies and how these precipitate star
formation [e.g., 155, 270, 330]. The rotation curves, tracing the
gravitational potential, provide important constraints on the (dark) matter
distribution and content [e.g., 304, 222]. Measuring kinematics is therefore
essential to constrain theoretical models of star formation, the formation
mechanism of disks at high redshift, the physical drivers of turbulence and
feedback [418, 212]. The ngVLA will be capable of performing sub-kpc scale
kinematics of cold neutral and molecular gas for star-forming galaxies during
the peak of cosmic star formation, and potentially the brightest first
galaxies at $z\geq 15$ via the fine structure lines, with a sensitivity that
is several times that of ALMA, as shown by simulations [61, see Fig. 9]. The
ngVLA will be able to trace disk dynamics to the large radii needed to
constrain the radial profile of both the rotation and velocity dispersion.
These measurements will provide important constraints on the dark matter
content of early galaxies and reveal the inner substructure and density
variations of galaxies (such as spiral arms, clumps, bars, rings), that are
key to understand the dynamical processes that are driving the formation of
stars during the peak epoch of cosmic star formation.

Figure 9: Simulated resolved kinematics in 20 h of observations for the ngVLA
compared to ALMA [61]. In all rows, the panels show the input model (left;
based on NGC 2366, convolved to the target resolution) and the simulated
observations with the ngVLA (center) and ALMA (right). The simulations assume
1.2 mJy (peak), 400 km s-1 line emission at 86 GHz, representing, for example,
CO(2–1) in a $z=1.7$ main-sequence galaxy or CO(3–2) in a starburst galaxy at
$z=3.0$. Top and middle: velocity integrated emission and velocity field,
respectively, at a resolution of 85 milli-arcseconds. The ngVLA can recover
the source (velocity) structure and surface brightness profile out to 0.4”
radius, while ALMA only recovers the bright peaks and structure in the inner
disk within 0.2”. Bottom: Position-velocity diagram along the major axis of
the galaxy. Several departures from pure disk rotation (e.g., the kink and
extra emission in the north part) are clearly visible in the ngVLA data but
not detectable with ALMA.

###  2.8 Detecting Supermassive Binary Black Holes with the ngVLA

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ ✓ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Silke Britzen Institution: Max-Planck-Institut für Radioastronomie, Auf
dem Hügel 69, Germany Email<EMAIL_ADDRESS>
Co-authors: E. Kun1,2,3,4,5, M. Zajaček6, A. Tursunov7, I.N. Pashchenko8,9

1 Theoretical Physics IV: Plasma-Astroparticle Physics, Faculty for Physics &
Astronomy, Ruhr University Bochum, 44780 Bochum, Germany; 2 Ruhr Astroparticle
And Plasma Physics Center (RAPP Center), Ruhr-Universität Bochum 44780 Bochum,
Germany; 3 Astronomical Institute, Faculty for Physics & Astronomy, Ruhr
University Bochum, 44780 Bochum, Germany; 4 Konkoly Observatory, HUN-REN
Research Centre for Astronomy and Earth Sciences, H-1121 Budapest, Konkoly
Thege Miklós út 15-17., Hungary; 5 CSFK, MTA Centre of Excellence, Konkoly
Thege Miklós út 15-17., Hungary; 6 Department of Theoretical Physics and
Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, 611 37
Brno, Czech Republic; 7 Max-Planck-Institut für Radioastronomie; 8 Astro Space
Center, Lebedev Physical Institute, Russian Academy of Sciences; 9 Moscow
Institute of Physics and Technology, Dolgoprudny, Institutsky per., 9, 141700
Moscow, Russia

#### Abstract

Galaxy collisions are observed throughout the Universe and the natural and
final consequences of their evolution are mergers of supermassive black hole
pairs detectable in gravitational wave emission with LISA or PTAs. Recently,
NANOGrav [6] provided first evidence for gravitational wave background
emission from ancient mergers. To detect, image, and dynamically explore the
precursors, the closest pairs of supermassive black hole binaries (SMBBHs)
with the anticipated and unprecedented superb capabilities of the ngVLA, is
the aim of the here presented project.

####  2.8.1 Precessing jets: The smoking gun signature of SMBBHs

The formation of SMBBHs in the aftermath of galaxy collisions has been
proposed by e.g., [22]. While black hole pairs on large scales have been
studied across the wavelength regimes e.g., [203, 209], direct imaging of
close pairs with high resolution radio interferometry remains elusive. To the
best of our knowledge, only one pair with 7.3 pc separation has so far been
imaged [333].

A mounting number of SMBBH candidates however, has been claimed based on radio
interferometric observations of precessing jets (e.g., [60, 51, 215]) . It is
the orbital motion of a supermassive black hole binary which leads to the
swirling motion of the jet (e.g., [22]). Precessing jets can also originate in
a misalignment of the black hole spin with the accretion disk angular momentum
which leads to the Lense-Thirring effect [223]. Precession-induced variability
is shown to not only cause pronounced curvature of blazar jets, but also to
modulate the variability due to periodically changing viewing angles and
Doppler factors (for details see [52] and references therein). Also the
Spectral Energy Distribution (SED) correlates with the projected precession
phase, as modeled for the most promising SMBBH candidate OJ 287 [52].

#####  2.8.1.1 Direct detection of absolute motion of the orbiting black hole
with the ngVLA

Based on the already existing archives of radio interferometric data (MOJAVE,
etc.) and our extensive experience in detecting, analyzing, and modeling of
precessing jets, we plan to select the best set of the closest pairs of SMBBH
candidates for future ngVLA observations. With the projected 0.1
milliarcsecond resolution and microarcsecond precision astrometry of the
ngVLA, we aim at direct imaging of both jets (if present) and cores, as well
as dynamically resolving the absolute motion of the orbiting core. As the
expected orbital timescale of the two supermassive black holes and hence the
radio cores is shorter than the precession timescale [52], we will also need a
temporal coverage of at least a fraction of the year. In Fig. 10 we show the
(circular) orbital speed of the $m_{1}$ and $m_{2}$ mass black holes in a
binary with a total mass of
$m=10^{6},~{}10^{7},~{}10^{8},~{}10^{9},~{}10^{10}~{}\mathcal{M}_{\odot}$
(orbital period is $T=1$ yr), where the mass ratio $\nu=m_{2}/m_{1}$
($m_{1}$$>$$m_{2}$) is set to $\nu=0.03$ and $\nu=0.3$ (the lower and upper
limit of the typical mass ratio range of merging SMBBHs). We show the results
for a range of redshifts. To translate the physical scale to angular scale, we
adopt the cosmological parameters as follows: $H_{0}=69.6$ km s-1 Mpc-1,
$\Omega_{\mathrm{m}}=0.286$ and $\Omega_{\Lambda}=0.714$. Microarcsecond
precision astrometric observations with the ngVLA, conducted on a time-scale
of years, might reveal the orbital displacement of at least the faster moving
black hole in a SMBBH - the secondary - given it is in active phase.

With the current Event Horizon Telescope (EHT), the future next generation EHT
(ngEHT), and in particular the ngVLA, never before seen radio inteferometric
observations will revolutionize direct imaging and astrometric motion studies.
These observations will be augmented by multi-wavelength observations by
upcoming new instruments (e.g, GRAVITY+, CTA). Together with LISA [11] and
Pulsar Timing Array observations in the Gravitational Wave regime, SMBBH
studies will likely open a new window for the exploration of the fundamentals
of gravity.

Figure 10: Orbital speeds of the black holes for a total mass between $10^{6}$
and $10^{10}$ M⊙ in a redshift range between 0.1 and 5.0, on logarithmic
scale. The plot on the left provides this information for a mass ratio of
1/30, the plot on the right for a mass ratio of 1/3.

###  2.9 Radio studies of the cosmic web

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ ✓ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Marcus Brüggen Institution: Universität Hamburg, Germany Email:
<EMAIL_ADDRESS>
Co-authors: M. Hoeft (TLS), D. Hoang (UHH), G. DiGennaro (UHH)

The cosmic web usually refers to the structure of galaxies arranged in
clusters, filaments, and voids that make up the large-scale structure of the
Universe. The synchrotron cosmic web refers to the corresponding synchrotron
emission that is produced in intergalactic magnetic fields by electrons
accelerated via shocks or turbulence. Studying the synchrotron cosmic web is
important to trace the missing baryons in the universe, and constrain
magnetogenesis, which refers to the origin and evolution of extragalactic
magnetic fields. Detecting magnetic fields in cosmic filaments and voids is
key to constrain magnetogenesis scenarios [362] because magnetic fields in
filaments are more pristine than the magnetic fields in galaxy clusters which
are affected by galactic winds and other processes.

Simulations predict and model this synchrotron emission [273]. While these
processes are fairly well studied in galaxy clusters, to date, observational
confirmation of the synchrotron web has remained elusive [389, 134, 235].
Stacking of all-sky radio maps has shown the polarization signature of the
synchrotron emission with polarization fractions $>$20% [392], but direct
detection will only be possible with the ngVLA, owing to its large sensitivity
to polarised continuum emission.

A related method to probe the magnetic fields in the cosmic web uses linearly
polarized radio sources to measure the field strength in thermal magnetized
plasma along the line of sight (RM grid). Recent studies have used RM grids
and their cross-correlation with large-scale structure to put upper limits on
the strength of the intergalactic magnetic field [277]. This has established
the RM grid as a powerful method to probe the magnetised cosmic web. The ngVLA
can provide a dense RM grid that covers a wide area and a broad range of
angular scales, allowing us to measure the strength of the intergalactic
magnetic field as a function of spatial scale and redshift.

Figure 11: Slice from a cosmological magnetohydrodynamical simulation showing
the synchrotron web. The red is the shocked total intensity radio emission.
The yellow color shows the temperature and gas density via the X-ray and SZ.
The blue lines show the vectors of the magnetic field lines. Simulation by F.
Vazza taken from Vernstrom et al. (2023).

Science goals: (i) Magnetic fields in filaments and voids via RM from
background sources.
(ii) Synchrotron emission from the cosmic web to study particle acceleration
and magnetic fields.

###  2.10 Galactic Structure

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ ✓ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Andreas Brunthaler Institution: Max-Planck-Institut für Radioastronomie
Email<EMAIL_ADDRESS>
Co-authors: Karl M. Menten, Max-Planck-Institut für Radioastronomie

Determining the structure of the multiple components of the Milky Way has been
a topic of active research for decades. Many of the early attempts relied on
kinematic distances that are not very reliable and even unusable in some parts
of the Milky Way. In particular the spiral arms are difficult to pinpoint
since they are typically obscured by pervasive interstellar dust whose high
extinction toward the spiral arms make them not accessible to optical
astrometry (i.e. Gaia) which also makes the use of photometric distances
challenging. However, Very Long Baseline Interferometry (VLBI) observations of
maser sources (mainly water and methanol masers) in high mass star forming
regions have reached accuracies that are comparable or even better than those
of Gaia parallaxes [55, 340]. Since these regions are predominantly found in
the spiral arms of a galaxy, these measurements enabled for the first time the
mapping of the spiral structure of large parts of the Milky Way as well as
providing accurate measurements of fundamental parameters of the Milky Way
such as its rotation speed and the distance to the Galactic center [158, 314,
313].

Recent advancements in the field of VLBI astrometry like MultiView and inverse
MultiView [327, 164], where multiple calibrators are used to get an improved
calibration of the disturbances from the atmosphere, have the potential to
improve the accuracy of parallax measurement by up to one order of magnitude
to about 1 $\mu$as. The ngVLA, with its multiple antennas at each of the long
baseline stations, will be suited perfectly to incorporate these new
techniques. Here, one antenna can observe the target source continuously,
while the other antennas observe the calibrators. Together with the
unprecedented sensitivity, the ngVLA will be able to measure parallaxes of
thousands of maser sources in high-mass star forming regions. This will give
access to weaker and more distant maser sources to map the spiral arms also on
the far side of the Milky Way, a region which will not be accessible to
optical surveys. Parallax measurements of masers with the ngVLA will therfore
be highly complementary to Gaia.

###  2.11 Chemical and physical conditions at the dawn of star and planet
formation

Thematic Areas: $\square$ Stellar Astrophysics $\square$ ✓ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Paola Caselli Center for Astrochemical Studies, Max Planck Institute for
Extraterrestrial Physics<EMAIL_ADDRESS>
Co-authors: Yuxin Lin, Center for Astrochemical Studies, Max Planck Institute
for Extraterrestrial Physics

The study of the initial conditions in the process of star and planet
formation, from cloud to protoplanetary disk scales, will greatly benefit from
the high sensitivity, spectral and angular resolution provided by the ngVLA.
As shown schematically in Figure12 (a), ngVLA will allow detailed mapping of
the inversion transition of ammonia, to study the connections of dense cores
(the units of star formation) to the surrounding cloud. [71] showed that high-
sensitivity and high-angular/spectral resolution are needed to disentangle the
core emission from that of the core, thus to trace such connection. This is
important to understand how dense cores form and evolve during the process of
star formation.

ngVLA will also greatly improve our understanding of the first steps toward
protostar and protoplanetary disk formation (see Figure12 (b)) as contracting
prestellar cores can be studied in detail from $\geq$10,000 au to $\leq$100 au
scales. This is important to compare with predictions from non-ideal MHD
simulations of star formation (e.g. [66]). Low-J transitions of deuterated
molecules, great tracers of cold and dense environments, can be mapped with
ngVLA ([110]). Ammonia inversion transitions can also be used to reconstruct
the physical and chemical structure of prestellar cores ([288]). Prestellar
cores also host the first steps toward molecular complexity (e.g. [16, 180,
179]), with emission typically confined in the outer layers of the cores (e.g.
[388, 148]). ngVLA will provide great sensitivity to image complex organic
molecules, building blocks of pre-biotic molecules, and significantly increase
their detection rate.

Moving from prestellar cores to protostars (see Figure12 (c)), ngVLA will
allow detailed mapping of the recently discovered streamers of material
connecting clouds to protoplanetary disks (e.g. [289, 381]). This will provide
a unique way to follow the asymmetric flow of material directly onto the disk
and study in detail the effect that such streamers have on the dynamical and
chemical evolution of disks, including the dawn of planet formation. Thanks to
the lower dust opacities at the ngVLA frequencies, complex organic molecules
can also be studied near protostars (e.g. [79]), thus allowing a fresh view of
the sublimated ices and more stringent tests on chemical modeling.

At protoplanetary disk scales, ngVLA will open a new window into the chemical
and structural details of disks, where the first steps of planet formation are
taking place (see Figure12 (d) and [346, 414]). Maps of complex organic
molecules will become available, thanks to the great ngVLA sensitivity, thus
shedding light on the link between disk and exoplanet atmosphere chemistry.
Last but not least, ngVLA will provide unique information on the inner zone of
protoplanetary disks, where terrestrial planets start to assemble (e.g. [85]).
Great synergy here is expected between ngVLA, JWST, and GRAVITY+.

Figure 12: Examples of projects that will greatly benefit from ngVLA: (a) the
cloud-core connection (the left panel shows the Mach number map obtained using
NH3(1,1) and (2,2) mapping of the Ophiuchus molecular cloud ([72]). (b)
Detailed physical and chemical structure of prestellar cores and the dawn of
protostars and protoplanetary disks (the left panel shows the deuterated
ammonia map at 3 mm obtained with ALMA ([65]). (c) The streamer connection
from cloud to disk scales (the left panel shows the 10,000 au scale streamer
discovered by [289]). (d) Physical/chemical structure of protoplanetary disks
and their "terrestrial zone" (the left panel shows the high-angular resolution
image of the youngest protoplanetary disk showing substructure (from [346]).

###  2.12 ngVLA studies of gravitational-wave triggers and other energetic
transient events

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Dominik Elsässer Institution: TU Dortmund Email: dominik.elsaesser@tu-
dortmund.de

Co-authors: Kevin Schmidt (TU Dortmund), Matthias Kadler (JMU Würzburg)

We propose to make energetic transient events, including the early stages of
the remnants of binary neutron star mergers, a scientific focus of a German
participation to the ngVLA. While these events via the association of the
LIGO-VIRGO event GW170817 with GRB170817A have convincingly been identified as
sources of short gamma-ray bursts and as sites of heavy element synthesis, our
understanding of the morphology and temporal evolution of the expanding
remnants of such collisions is severely lacking. This is especially true for
the majority of events where no forward-beamed high-energy emission is
observed. Radio-interferometric observations and high frequencies and extreme
sensitivities, as offered by the ngVLA, could offer a unique observational
window to those processes, and thus – in close concert with gamma-ray
observations e.g. with the CTA – also contribute to elucidating their
potential role as cosmic ray acceleration sites. It is clear, given the large
distances to almost all of these sources, that a direct resolution of the
morphology, and direct identification of a putative relativistic outflow, may
be highly demanding and at times not even feasible. Thus, as a first step, we
aim to examine the observational data for apparent movements of the emission
origin, which would be a strong indicator of relativistic outflows occurring,
even for sources where there is negligible forward boosting in the direction
of our line of sight, or even de-boosting, and where the source itself is not
spatially resolved. This would be a case where, although the source itself
would appear to be point-like, the highest possible resolutions are required.
For this reason, we propose observations in the cm wavelength range, which
corresponds to a frequency range of around 25 GHz. Identification of an
apparent non-stationary and time-variable point source would then indicate
movement of the emission site, potentially identified with shocks in a
relativistic outflow.

We aim to embedd our observations in a multi-wavelength context, and to
combine them with high-sensitivity observations from gamma-ray observatories
like the MAGIC telescopes and the Cherenkov Telescope Array (CTA), where the
proposers are long-term collaboration members. Transients are a central part
of the CTA’s observing program [375, 63], and high-impact observational data
from prototype telescopes is expected already in the next few years [62]. It
is thus timely to think about an ideal approach to include a German
participation to the ngVLA into such a strategy.

To analyze the observations, we propose an innovative analysis method based on
deep learning techniques. The radionets project [342] focuses on the benefits
neural networks can bring to analyzing radio interferometer data. Radionets
cleans up the incompletely calibrated radio interferometer data by
reconstructing missing information directly in Fourier space. This unique
approach not only delivers high-resolution images faster, but also tends to
produce fewer artifacts in the cleaned images compared to conventional
analysis approaches [124]. This can prove especially beneficial in light of
the scientific question posed above, a search for apparent motion in faint
point-like, or near point-like, sources. This deep learning approach is
presently being further developed to incorporate information from other
frequency bands in a meaningful way to create the imaging product.

Furthermore, we will research machine learning methods as developed e.g. in
the Lamarr center for outlier detection to facilitate rapid and reliable
transient detection in the observational data.

###  2.13 Hyper Compact Radio sources in the Galactic Center

Thematic Areas: $\square$ ✓ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ ✓ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Sebastiano von Fellenberg Institution: Max Planck Institute for Radio
Astronomy, Bonn Email<EMAIL_ADDRESS>
Co-authors: Arman Tursunov, Andrei Lobanov, Gunther Witzel

Stellar Remenants in the Galactic Center – A large number of stellar remnants
in expected to be found in the Galactic Center [e.g., 263, 142, 122], which is
trivially explained by the large number of massive stars currently observable
in the Galactic Center today (e.g., [393]). Of these stellar remants, at least
one bonafide stellar remnant has been detected, that is the Magnetar SGR
J174-2900 located just 3” from Sgr A* [194, 262, 312].

A census of Hyper Compact Objects – [417] conducted a deep $33~{}\mathrm{GHz}$
and $44.6~{}\mathrm{GHz}$ JVLA survey of the central arcseconds and identified
68 hyper-compact objects in the Galactic Center (see Figure 13). The nature of
these objects is not finally determined, however, the authors categorized the
objects into three categories based on their spectral properties.

  * •

Flat-spectrum HCRs: [417] speculated that these sources may be unresolved
peaks in HII regions because their characteristic emission profile matches
that of the free-free emission.

  * •

Inverted-spectrum HCRs: [417] argue that two scenarios are likely: The spatial
correlation indicates these sources are due to strong stellar winds.
Nevertheless, the spectral similarity of these sources with the spectrum of
the GC magnetar indicates that a fraction of the sources may be other
magnetars.

  * •

Steep-spectrum HCRs: These sources have spectral indices that indicate none-
thermal emission. [417] argue that this implies that these sources are most
likely associated with massive stellar remnants.

The ngVLA will drastically improve the sensitivity compared to JVLA, and we
thus expect a significant increase the number of detectable HCRs. If these
sources can be robustly identified as the radio counterparts of compact
stellar remnants such as black holes or neutron stars, their measurement will
allow to derive the first direct constraints of the dark mass distribution
close to a super massive black hole.

Astrometry of HCR-sources – Due the extreme senstivity of the ngVLA, we expect
to achieve nano-arcsecond astrometry, implying that these objects can be
followed with extreme astrometric precssion.

Due to the likely absence of emission lines for these sources, astrometric
information alone is insufficient to determine orbital solutions. Thus these
objects cannot directly be used as gravitional probes. They still offer the
chance of being used as for parallax studies, which is on the order of
$1/8200pc=x[^{\prime\prime}]\sim 125~{}\mathrm{\mu as}$. The parallax may thus
yield an independent distance measurement to the Galactic Center, similar to
the maser stars used traditionally (e.g., Reid & Brunthaler 2020).

Binary-systems – Due to graviational in-spiral and the Hills mechanism, binary
systems are not stable close to Sgr A*. At larger distances, binaries can
survive for a long time and are thus expected to be found. Two binary systems,
consisting of at least one star have been identified so far [123]. HCR sources
may represent the radio-detectable counterpart of NS or BH-binary system,
ngVLA observations offer the chance for an astrometric detection of such
binaries. Depending on the separation distance, binary systems can be divided
into close and wide binaries. The former are often associated with transient
phenomena and as gravitational wave (GW) emitting sources, allowing one to
test gravity in a strong regime.

Direct observation of binary systems in the Galactic center could be important
in the context of future GW telescopes, such as LISA, providing tighter
constraints on the expected rate of GW events and observing the GW emitting
binaries prior to merger. In this sense, in addition to the NS-BH binary
studies, mentioned above, the systems without NS, but involving black holes
and white dwarfs are of a special interest. According to [397] BH-BH binaries
and those involving WD provide the most plausible systems observable by future
LISA GW experiment.

Figure 13: JVLA map of the Galatic Center. Green crosses show the location of
hyper-compact objects as classified by [417].

###  2.14 Substructures of dust rings in protoplanetary disks

Thematic Areas: $\square$ Stellar Astrophysics $\square$ ✓ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Mario Flock Institution: Max-Planck Institute for Astronomy Email:
<EMAIL_ADDRESS>
Co-authors: Takahiro Ueda, MPIA, Til Birnstiel, LMU, Sebastian Wolf, Uni Kiel,
Myriam Benisty, IPAG Grenoble

Since the revolution of high-angular resolution observations at
submillimeter/millimeter wavelengths of protoplanetary disks we have learned,
their morphology is made of azimuthally symmetric or point-symmetric
substructures, in some cases with spiral arms or localized spur- or crescent-
shaped features.

Even the majority of theoretical studies to explain the observational results
have focused on disk models with embedded planets, only in very few cases have
exoplanets been detected in these systems.

Furthermore, the concentration of solid material at pressure traps are
expected to appear before planets form, as they are necessary to drive the
concentration of small solids which can lead to the formation of
planetesimals.

In the work series [107, 29] we present observational predictions from high-
resolution 3D radiative hydrodynamical models that follow the evolution of gas
and solids in a prototoplanetary disk. We focus on substructures in the
distribution of millimeter-sized and smaller solid particles produced by the
vertical shear instability.

Figure 14:  Left: Image of the ALMA dust-continuum emission at 1.3 mm obtained
with observational characteristics that resemble those obtained for the ALMA
DSHARP project. The resultant rms noise on the map and synthesized beam, shown
in the lower left corner, are about 15.0 $\mu$Jy beam-1 and 43 mas $\times$ 28
mas, respectively. Right: Simulated long ngVLA observations for the dust
emission at 3 mm for our face-on disk model. The resultant rms noise and the
synthesized beam, correspond to about 0.1 $\mu$Jy beam-1 and 8 $\times$ 6 mas,
respectively.

We show that their characteristics are compatible with some of the shallow
gaps detected in recent observations at sub-mm/mm wavelengths and present
predictions for future observations with better sensitivity and angular
resolution with ALMA and a Next Generation Very Large Array.

###  2.15 Probing SgrA⋆ flares with the ngVLA

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ ✓ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Authors: Christian M. Fromm Institution: JMU Würzburg, Würzburg,
Germany Email<EMAIL_ADDRESS>
Co-authors: Luciano Rezzolla (Inst. für Theor. Physik, Goethe Universität,
Frankfurt, Germany), Michael D. Johnson (CfA Harvard & Smithsonian, USA)

The supermassive black hole (SMBH) in the centre of our Milky Way, SgrA⋆,
represents a perfect laboratory for fundamental and plasma physics [131].
Observations with the GRAVITY experiment provided a mass value of $4.14\times
10^{6}\,M_{\odot}$ [136]. However, the spin of the black hole, $a^{\star}$,
and its inclination, $\vartheta$, are less constrained. The recent Event
Horizon Telescope (EHT) observation shows a ring like structure and the best
bet models are characterised by $a^{\star}\geq 0.5$ and an inclination
$\vartheta\leq 30^{\circ}$ and favour a jet-dominated emission model [104]. In
addition, SgrA⋆ is well known for its flaring behaviour in the near infrared
and in the x-ray regime [135, 405]. However, the detailed mechanism for these
flares are still on debate.
A promising model for the flares in the galactic centre is related to magnetic
re-connection, i.e., the re-configuration of the magnetic field topology while
energizing the radiative particles. Recent Particle in Cell (PIC) simulations
have shown that during the re-connection events electrons can be accelerated
from a thermal Maxwell-Jüttner distribution into a non-thermal distribution
[18, 255] which could be connected to the SgrA⋆ flares. Global general
relativistic magneto-hydro-dynamical (GRMHD) simulations of accreting black
holes have identified the equatorial current sheet generated during the
accretion process as possible location for magnetic re-connection events. This
current sheet is in-stable against the tearing instabilities and lead to the
formation of plasmoids, i.e. magnetic islands, while heating the particles
[268, 329, 175]. The formed plasmoids orbit as coherent structure the black
holes and are sheared out over time in the larger accretion flow [269]. In
addition to the plasmoids generated in the equatorial current sheet plasmoids
can also be generated in the shear layer between the jet and wind region due
to Kelvin-Helmholtz (KH) instabilities. Some of those KH-driven plasmoids grow
in size while gaining energy and finally become unbound by the black hole
[268]. Based on our simulations we could track large-scale plasmoids out to a
radius of $r\sim 100$ gravitational radii [268].

Indeed, the observed GRAVITY flares in the infrared could be associated with
motion of plasmoids [137]. Thus the question arises whether we can detect the
formation of plasmoids and further investigate the physical mechanism of the
SgrA⋆ flares with future observations. The ngVLA provides an unprecedented
coverage of the u-v plane, high frequency observing capabilities from 70 GHz
to 116 GHz together with improved
sensitivity666https://ngvla.nrao.edu/page/performance. Given that the
characteristic time scale of the galactic centre is $\sim 20\,s$ standard
imaging of several hours long VLBI observations will merge several orbital
periods of the plasmoids and their signature will most likely smeared out.
However, the impressive u-v coverage of ngVLA will allow for snapshot-
imaging777see for example
https://library.nrao.edu/public/memos/ngvla/NGVLA105.pdf, i.e, the splitting
of the observations into several short time bins during the imaging process,
and thus mitigate the variability of SgrA⋆ during the imaging. Given the
common flaring activity of the galactic centre observing campaigns of several
consecutive days with the ngVLA will most likely catch a flare and thus allow
us test the hypothesis of magnetic re-connection driven outburst.

Figure 15: Properties of a plasmoid chain formed in GRMHD simulations of a
black hole with $a^{\star}=0.94$. The panels show from left to right the
azimuthal component of the magnetic field, $B_{\phi}$, the ratio of pressure
to density and the Bernoulli parameter, $-hu_{t}$, indicating bound and
unbound plasma. The formed plasmoids are clearly visible in the left most plot
[268].

To summarize, the ngVLA with its distinguishing features mentioned above
provides the unique opportunity to test the formation of plasmoids during
magnetic-reconnection events and their possible connection to the flaring
behaviour of SgrA⋆. Therefore, future ngVLA observations of SgrA⋆ in close
combination with state-of-the art numerical modelling will allow us to provide
answers to one of the most pending questions regarding the black hole in our
Milky Way.

###  2.16 Jet Launching and Radiation Microphysics

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Christian M. Fromm Institution: Institute for Theoretical Physics and
Astrophysics, JMU Würzburg, Emil-Fischer Str. 31, 97074 Würzburg, Germany
Email<EMAIL_ADDRESS>
Co-authors: Anne-Kathrin Baczko (Chalmers Univ. Gothenburg, Sweden), J. Anton
Zensus (MPIfR), Luciano Rezzolla (Inst. für Theor. Physik, Goethe Universität,
Frankfurt, Germany)

Relativistic jets are the most prominent and fascinating feature in Active
Galactic Nuclei (AGN). They are anchored in the nucleus of AGNs, propagating
as a collimated outflow several thousands of light years through their host
galaxy before finally encountering the interstellar medium. According to our
current understanding, relativistic jets are powered either by the potential
energy released during the accretion of matter from the surrounding disk onto
supermassive black holes (SMBHs) or by directly tapping the rotational energy
of the spinning SMBH [34, 31]. In both scenarios magnetic fields play a
crucial role. Yet, its origin and geometry is unclear.

Most of our knowledge on relativistic jets is inferred from Very Long Baseline
Interferometric (VLBI) observations which can resolve their structure from
their outskirts down to the horizon scales [105, 99]. Given the advances in
numerical methods and available computational resources the launching of
relativistic jets from black hole accretion disk systems can be modelled using
General Relativistic Magneto-Hydrodynamic (GRMHD) simulations [see 258, for a
review]. Despite the substantial observational and theoretical progress in
understanding jets many fundamental questions regarding their formation,
acceleration and collimation remain unanswered [see 30, for a review].
The recent 86 GHz observations of M 87 using the Global Millimetre VLBI array
detected both, a ring like inner structure and a launched large scale jet
providing the opportunity to observe and model both main features
simultaneously [237].
The ngVLA will provide both, an unprecedented u-v coverage, a wide frequency
coverage (1 GHz-116 GHz) and improved sensitivity. These unique capabilities
will allow us to improve not only the observations of Lu et al. [237] but also
observations of relativistic jets in AGNs in general. We can further
investigate i) the magnitude and geometry of the magnetic field in the direct
vicinity of SMBHs and ii) possible particle acceleration mechanisms.
Performing ngVLA multi-frequency observations of AGNs and using modern image
reconstruction algorithms [67] the distribution of the turnover flux density,
$S_{\mathrm{t}}$, and frequency, $\nu_{\mathrm{t}}$, can be obtained. From the
turnover values the magnitude of the magnetic field and the particle density
can be computed [111, 76, 112]. The geometry of the magnetic field can be
extracted from possible polarisation observations which will also benifit from
the improved observing capabilties of the ngVLA. In addition to the turnover
values, the spectral slope will be obtained from the multi-frequency
observations. Comparing the distribution of the spectral index along the jet
with advanced GRMHD simulations [259] will provide details on the particle
acceleration mechanism and side. Figure 1 shows the distribution of the
turnover frequency, $\nu_{\mathrm{t}}$, turnover flux density,
$S_{\mathrm{t}}$, and optically thin spectral index, $\alpha$ from a 3D GRMHD
simulations of M 87 for a thermal particle distributions (top) and for a kappa
distribution (bottom) [112].

Figure 16: Turnover values from a 3D GRMHD simulation for M87 including
magnetic reconnection heating. Distribution of the turnover frequency,
$\nu_{\mathrm{t}}$, the turnover flux density, $S_{\mathrm{t}}$, and optically
thin spectral index, $\alpha$, for a thermal particle distribution (top) and
non-thermal (kappa) particle distribution (bottom) [112]

.

To summarize, future ngVLA observations in close combination with GRMHD
simulations will shield light on the disk-jet connection in AGNs and on the
acceleration of particles in relativistic jets and will provide answers to the
pending questions of AGN and jet physics.

###  2.17 Galaxy clusters with ngVLA

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ ✓ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Matthias Hoeft Institution: Thüringer Landessternwarte Email: hoeft@tls-
tautenburg.de

Co-authors: M. Brüggen (UHH), Aritra Basu (TLS)

Radio emission from galaxy clusters is an important probe of cosmic
magnetogenesis, plasma physics, and the interaction of active galaxies with
their environment [92, 380, 383, 120, 365, 216]. Knowing the properties of the
intra-cluster medium and studying its evolution is crucial for determining how
magnetic fields are amplified during cosmic evolution, for exploring the
physics of particle acceleration in the very low density environment, and for
solving the puzzle of how the feedback from the AGN in the central galaxy in a
relaxed cluster regulates its accretion from the intra-cluster medium [154].

Radio emission probes in particular the non-thermal component in clusters,
namely the magnetic field and cosmic ray electrons. Clusters show a rich
variety of outbursts from the central galaxies, which are thought to
compensate for the cooling of the central intra-cluster medium, self-
regulation of accretion and feedback, perturbed AGN, stripped (jellyfish)
star-forming galaxies, and diffuse emission features, relics and halos, which
are thought to trace merger shocks and merger-induced turbulence. Neither the
acceleration of electrons to relativistic energies nor the strength, structure
and evolution of cluster magnetic fields are well understood.

Radio observations in L-, S- and C-band have been shown to shed light on CRe
and magnetic fields, e.g. when using background sources to study the RM of the
ICM, the sources could be largely depolarised at low frequencies. The same is
true for emission AGN, radio galaxies, relics embedded in the ICM, depending
on the amount of ICM in front of these sources, they may be depolarised at low
frequencies. Another crucial aspect is the broad-band study of the morphology
of the sources, which is necessary to understand to what extent the
filamentary structure may directly reflect the magnetic field structure of the
ICM. Very broadband, sensitive, spatially resolved observations are essential
to determine the complex structure of the ICM in Faraday space.

Figure 17: Polarization intensity images of the radio relic and AGN in the
galaxy cluster MACS J0717.5+3745 at 2 arcsecond resolution, showing that the
polarization emission is distributed in a clumpy manner. The image also revels
fine, small-scale filaments visible in the total power emission. Contour
levels are drawn at $[1,2,4,8,...]\times\sigma_{\rm rms}$ and are from the VLA
L-, S-, and C-band Stokes I images. The beam sizes are indicated in the bottom
left corner of the each image. The image has been published in Rajpurohit et
al. (2022) [308].

The scientific objectives of the ngVLA would be: (i) to use very broadband
spectro-polarimetric observations to study the magnetic field structures. (ii)
measure the spectral properties up to the highest frequencies to constrain the
acceleration processes in the intra-cluster medium.

###  2.18 Resolving the Doppler Crisis

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Matthias Kadler Institution: JMU Würzburg Email:
<EMAIL_ADDRESS>
Co-authors: Eduardo Ros (MPIfR)

Blazars are a subclass of AGN that emit violently variable broadband emission
from radio to $\gamma$-ray energies. Understanding their physics is of very
high interest for astroparticle physics as these objects are possible sources
of ultra-high-energy cosmic rays and neutrinos [e.g., 153, 245]. Since the
1990s, Imaging Air Cherenkov Telescopes have been highly successful in
detecting gamma-ray emission up to TeV photon energies from an increasing
number of blazars. Already today, about 80 TeV blazars are known and it has
been demonstrated that TeV observations can efficiently be combined with radio
single-dish and VLBI results to probe source emission models [189, 9].
However, the majority of all known TeV blazars are high-synchrotron-peaked
(HSP) objects of the BL Lac type which can be detected at TeV energies
typically only during flaring states and which are faint radio sources. A
major improvement will be achieved in the next decade with the Cherenkov
Telescope Array [CTA; 4] thanks to its sensitivity and low energy threshold
[86]. With the improved flux sensitivity of CTA, it will be possible to study
a large population of blazars both in high and low states and measure their
high-energy spectral variability. In combination with the next generation Very
Large Array (ngVLA; [266]), it will become possible to solve current major
questions regarding the high-energy emission of blazars.

One of the most pressing questions in this context is the so-called "Doppler-
crisis" [150]. Usually, the highly variable gamma-ray emission of blazars is
explained by very high Doppler factors up to $\delta>40$ [e.g., 371]. In stark
contrast to this, radio observations have found that HSP jets typically move
at comparatively low speeds and show relatively low brightness temperatures,
indicative of low Doppler factors. One possible explanation is that the radio
emission and the variable $\gamma$-ray emission may not originate from the
same emitting region. This could be explained by the existence of a "spine-
sheath" structure [125] where the outer layers of the relativistic jet (the
sheath) have a slower bulk velocity along the jet axis than the inner layers
(the spine) and can serve as a seed-photon field for inverse-Compton processes
within the latter. Other scenarios involve the natural presence of stationary
recollimation shocks in jets [e.g., 151]. Such models can be constrained by
high-resolution VLBI observations but previous observations have only yielded
rather low signal-to-noise data [291] or studies of the very brightest
individual sources at low frequencies [129]. The vast majority of the HSP
blazar population is much too faint for current-generation VLBI instruments.
With the ngVLA, it will become possible to resolve different emission regions
in structured blazar jets both in total intensity and in polarized emission at
unprecedented angular resolution and sensitivity. This will yield the key data
to test multi-zone broadband emission models towards a solution of the Doppler
crisis.

Figure 18: VLBA image at 8 GHz of a typical faint TeV blazar (1ES 0347$-$121;
[290]; left) and High-Sensitivity-Array image at 1.4 GHz of the brightest HSP
blazar (Mrk 501; [129]; right). The ngVLA will provide the image sensitivity
needed to detect regions of low surface brightness and measure their
polarization structure for the bulk HSP blazar population. This is currently
possible only for the very brightest sources and only at low frequencies in
long experiments.

###  2.19 VLBI Probes of Neutrino Emission in Blazars

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Matthias Kadler Institution: JMU Würzburg Email:
<EMAIL_ADDRESS>
Co-authors: Yuri Kovalev (MPIfR), Eduardo Ros (MPIfR)

The origin of the high-energy cosmic neutrinos detected by the IceCube
observatory is a hotly debated topic in astroparticle physics with growing
evidence that some of these neutrinos may be associated with active galactic
nuclei (AGN), in particular blazars. In 2017, an association of the track-like
muon neutrino event IC 170922A with the $\gamma$-ray blazar TXS 0506+056 has
been found at a significance of $\sim 3\,\sigma$ [165]. An independent second
analysis found a $3.5\,\sigma$ significant association between the TXS
0506+056 blazar and a so-called ’flare’ of lower-energy neutrinos in 2014/15
[166] during a low $\gamma$-ray emission state. This surprising apparent lack
of $\gamma$-ray emission during at least some episodes of enhanced neutrino
production has been widely discussed in the literature [e.g., 315, 332, 214,
252, 213]. A leading idea is that particle cascades could lead to a
redistribution of the calorimetric output toward lower energies. This can
eventually lead to strong shocks on parsec scales and explain associated radio
variability. The radio band is thus of utmost importance for a better
understanding of neutrino emission in blazars and for the related major puzzle
regarding the origin of high-energy cosmic rays [153].

Statistical correlations have been claimed by multiple groups [296, 159, 297,
298, 58, 167, 57, 23, 366] but recent studies also revealed the difficulties
and limitations of matching large source samples with neutrino events [e.g.,
2]. Here, we focus on the complementary approach to investigate individual
sources with novel high-quality observational data with the aim to understand
the neutrino emission mechanisms of blazars.

In hadronic emission models, the high-energy $\gamma$-ray emission of blazars
is produced by interactions of relativistic protons in the jet with soft
ambient seed photons [e.g. 244] and neutrino emission is naturally expected.
The origin and properties of the seed photon field are a major open question.
An attractive model [372, 370] assumes that the high-energy protons of the
fast inner spine of the blazar jet interact with soft target photons of the
sheath, a slower jet-layer surrounding the spine, leading to high-energy
$\gamma$-ray and neutrino emission. Other models suggest that neutrino
production could happen closer to the jet base, e.g., in standing
recollimation shocks [e.g., 190]. VLBI observations can probe such models if
sufficient angular resolution, sensitivity and image fidelity are provided.
The putative spine and sheath structures are likely to have different
magnetic-field configurations so that polarization information is of high
relevance.

Previous VLBI observations of neutrino-candidate blazars have indeed found
indications of limb-brightened structures, as predicted in spine-sheath models
[275, 192, 335] albeit at limited image fidelity so that the candidate regions
for the photopion-process seed photon fields cannot be fully resolved and
characterized. Fig. 19 shows 15 GHz MOJAVE images of three neutrino-candidate
blazars, which demonstrate the need for higher resolution. This is generally
achieved at high frequencies and since a good $(u,v)$ coverage is driving
image fidelity, the next generation Very Large Array (ngVLA; [266]) will be
superior to other VLBI instruments in this science context.

Figure 19: Spine-sheath structures seen at 15 GHz (VLBA, MOJAVE): TXS 0506+056
(left), PKS1502+106 (middle) and PKS 1741$-$038 (right). EVPAs at the outer
edges of the jets (the sheaths) are near-perpendicular to the inner part (the
spine). For each source, the left sub-panel shows total intensity contours and
fractional linear polarization in color. The right sub-panel shows the
$3\,\sigma$ total-intensity contour and inner contours of linear-polarized
intensity with EVPAs superimposed. Beam sizes and scale bars are shown in the
bottom left and right corner.

###  2.20 Low-$J$ CO Line Excitation

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ ✓ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Ryan P. Keenan Institution: Max-Planck-Institut für Astronomie Email:
<EMAIL_ADDRESS>
Co-authors: Leindert A. Boogaard (MPIA), Fabian Walter (MPIA), Melanie
Kaasinen (ESO), Dominik Riechers (Uni Köln), Nikolaus Sulzenauer (MPIfR)

Molecular hydrogen (H2) is the fuel for star formation. Because the abundance
of H2 is difficult to measure directly, rotational emission from carbon
monoxide (CO), the second most abundant interstellar molecule, is the
preferred tracer of molecular gas abundance. The fundamental $J=1\rightarrow
0$ transition of CO (hereafter CO(1–0)) is easily excited under conditions
prevalent in molecular clouds and has been carefully calibrated as a molecular
gas tracer [36]. Transitions amongst the higher energy rotational states of CO
form a ladder of emission lines which trace conditions of increasing gas
density and temperature. The relative intensities of multiple CO lines can
therefore be used to derive the density and temperature molecular gas clouds
[77, 191, 40, 382, 229], which may provide new insight into how gas abundance,
ISM conditions and stellar feedback couple to regulate star formation and
drive the evolution of the galaxy main sequence.

The lowest energy transitions are critical to such measurements, as they are
sensitive to cold, diffuse gas components, which may dominate the total
molecular gas mass but do not contribute significantly to luminosity the
$J_{u}\gtrsim 3$ lines [191]. Figure 20 shows a compilation of low-$J$ CO
spectral line energy distributions (SLEDs) from [77], [218] and [193].
Galaxies exhibit a wide range of line intensity ratios, and a significant
fraction have SLEDs which cannot easily be reconciled with simple models of
molecular clouds, motivating the need for more sophisticated models.
Interestingly, while $z\sim 0$ line ratios show evidence for correlation with
star formation rate surface density, not all high-$z$ galaxies match this
behavior, suggesting that the coupling between molecular cloud properties and
star formation rate is evolving over time.

The current generation of sub/millimeter facilities are capable of measuring
the low-$J$ CO lines only in small samples of high-$z$ galaxies, typically by
combining VLA observations of CO(1–0) with ALMA or NOEMA observations of the
higher-$J$ lines [77, 319]. Small sample sizes limit the power of current
studies to map out statistical trends in $z>1$ CO SLEDs and ISM properties.
The ngVLA will be $\sim 5$ times more sensitive than the VLA between 10 and 50
GHz and ALMA between 70 and 115 GHz. This results in a factor of 25 reduction
in observing time for CO(1–0) at $10>z>0$ as well as CO(2–1) at $z>1$ and
CO(3–2) at $z>3$, and will enable measurements of the lowest $J$ transitions
for representative samples of high redshift galaxies.

The resolution of the ngVLA will also enable resolved mapping of CO line
ratios at $\sim$kpc scales. This will make it possible to study the evolving
effects of galactic environments like bars and active nuclei, which are known
to affect gas conditions in $z\sim 0$ galaxies [200, 83, 411] and likely play
an important role at high redshift as well [363].

Figure 20: Left: The $J_{u}=1$–$3$ SLEDs, normalized by CO(2–1) flux, for a
samples of $z\sim 0$ (circles) and $z\sim 1.5$ (stars) galaxies are shown,
color coded by star formation rate surface density. Right: CO(2–1)/CO(1–0) and
CO(3–2)/CO(2–1) flux ratios for the same sample are shown, compared to a grid
of cloud models from [224]. Galaxies show a diverse array of SLEDs. For local
galaxies, the line ratios, which are sensitive to the density and temperature
of the emitting gas, correlate with properties such as SFR surface density,
pointing to a coupling between the bulk conditions in molecular clouds and
their ability to form stars. However the small sample of $z\sim 1.5$
observations do not uniformly obey these trends, providing tentative evidence
that this connection is evolving over time. Larger samples of $z>1.0$ CO
SLEDs, only possible with the ngVLA, are necessary to robustly explore this
possibility.

###  2.21 Radio-optical link: AGN-based VLBI and Gaia fundamental reference
systems and their application for astrometry and astrophysics

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Yuri Kovalev Institution: MPIfR, Bonn Email<EMAIL_ADDRESS>
Co-authors: Eduardo Ros (MPIfR), Andrei Lobanov (MPIfR)

The most recent results of the Gaia astrometry and photometry information is
summarized in the Gaia DR3 catalog [115]. Through cross-identification with
radio-bright active galactic nuclei (AGN) hosting parsec-scale jets888Radio
Fundamental Catalog (RFC): http://astrogeo.smce.nasa.gov/rfc, several thousand
matches have been discovered. Approximately 10% of these matches exhibit
significant positional VLBI-Gaia shifts along their jets, primarily due to the
extension of optical jets while VLBI pinpoints the compact parsec-scale core
(Figure 21). Opacity-driven frequency-dependent core-shift and extended radio
structure also plays some role in this effect. Additionally, it has been
deduced that Gaia positions significantly fluctuate due to optical flares
occurring in their cores. Details can be found in [206, 286, 285, 287, 300,
208]. Optical and VLBI AGN data applied together can help to differentiate
between flares occurring in accretion disks (which are expected to make the
spectrum more blue since the accretion disk thermal emission peaks in UV) and
synchrotron flares in the jet. This joint analysis they will allow us to
dissect the disk-jet system in AGN.

ngVLA will provide continuous frequency coverage and significantly improved
fringe detection sensitivity, image dynamic range and image fidelity. As a
result, the sample of AGN with detectable jets will increase by more than an
order of magnitude with significantly enhanced accuracy of the absolute
astrometry measurements. This will deliver the next generation of the radio
fundamental reference frame and provide grounds for a transformational change
in our ability to study the accretion disk – relativistic jet interaction in
active galaxies addressing questions of jet formation and energy extraction.

Figure 21: Left: Diagram explaining the definition taken for the two preferred
VLBI-Gaia offset directions with respect to the parsec-scale jet: downstream
$\Psi=0^{\circ}$ and upstream $\Psi=180^{\circ}$. Right: A distribution of the
angle $\Psi$ between the VLBI-Gaia positional shift direction and direction of
VLBI jet filtered on its error value. Gaia DR3 optical and RFC radio positions
are used, jet position angles are taken from [301].

###  2.22 Blazars as neutrino candidates: complete sample studies

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Yuri Kovalev Institution: MPIfR, Bonn Email<EMAIL_ADDRESS>
Co-authors: Matthias Kadler (JMU Würzburg)

Currently, studies of high-energy astrophysical neutrinos and their sources
are limited by the sensitivity and resolution of neutrino observatories. A new
era is coming, as their capabilities are increasingly improving. The next-
generation IceCube-Gen2 will grow the telescope volume tenfold, from 1 to 10
cubic kilometers, aiming at a corresponding increase in detection rates by
2033 [1]. At the same time, it is expected that water-based detectors will
provide a better angular resolution than the current IceCube, both for muons
and for cascades. The Baikal-GVD detector has already reached the instrumental
volume of 0.5 cubic kilometer and continues to grow and improve event
reconstruction algorithms [10]. KM3NeT, a neutrino observatory in the
Mediterranean, is being constructed and has already started yielding its first
results [7]. Together, these instruments will provide a qualitative leap
forward in both the number of detected astrophysical neutrinos and in the
precision of their localization.

While neutrino telescopes continue to gather more neutrino detections, radio
monitoring of AGN before, during, and after these events is crucial for
identifying the potentially neutrino-associated changes in their compact jet
structure. Given the current small number statistics, one cannot predict which
AGN will be positionally associated with future neutrinos. Thus observing a
large, complete candidate sample is necessary to conduct a wide range of
statistical analyses [e.g., hovatta2021, 2022A&A...666A..36L, 298]. We note
that studying a complete sample of AGN with well-defined properties will allow
us not only to relate the observed changes to detected neutrinos but also to
set a robust significance on that association. Brighter blazars are more
likely counterparts to detected neutrinos, as has been specifically
demonstrated by Plavin et al. [299]. This motivates selecting AGN by their
parsec-scale flux density in order to maximize the probability of obtaining
associations.

While the original claims of the association are confirmed when the same data
are used, newer reconstructions of IceCube data or different selection
procedures may result in the apparent reduction of the association
significance [2]. This trend was reported for the original TXS 0506$+$056
association by Luszczak [240] as well as for various blazar statistical
associations by Bellenghi et al. [23]. This additionally supports continuous
VLBI monitoring of complete samples to provide grounds for future analysis
against evolving neutrino data.

ngVLA with its enhanced sensitivity and observing efficiency will allow for
such monitoring to study conditions in and around AGN cores before during and
after high energy neutrino arrival. Its highest observing frequency of about
100 GHz will allow us to study regions close to the central engines mitigating
synchrotron opacity. This opens exciting opportunities to understand particle
acceleration and neutrino production.

###  2.23 Dense gas in nearby galaxies

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ ✓ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Kathryn Kreckel Institution: Heidelberg University Email:
<EMAIL_ADDRESS>
Co-authors: Frank Bigiel (Univ. Bonn), Eva Schinnerer (MPIA)

The physical processes that drive the formation of stars inside galaxies is
regulated by the cosmic matter cycle, and operates on the sub-100 pc scales
defined by the sizes of individual giant molecular clouds and the radiative
and mechanical reach of stellar feedback. In recent years, the old idea of a
universal population of Giant Molecular Clouds (GMCs) has been replaced by a
more diverse, dynamical view (e.g., [160, 253]) based on clear evidence that
cloud properties (e.g., gas mass surface density, line width, internal
turbulence) are shaped by their environment (e.g., [73, 364]). Pinpointing
these correlations requires an external view of complete galaxies, but the
distance to which the relevant scales are accessible by current facilities is
limited. The ngVLA has the potential to directly map molecular gas across a
much larger sample of nearby galaxies, achieving 50 pc scales out to 100 Mpc
distances.

Tracing dense gas in extragalactic targets is challenging with today’s
instruments as lines (e.g N2H+ [178]) and line ratios (e.g. HCN/CO [177] or
N2H+/HCN [178]) are simply too faint to be mapped over large parts of nearby
galaxy disks and remain challenging even with ALMA. Extragalactic observations
must use the brightest higher-critical density lines, such as HCN, HCO+, and
HNC where ratios between lower and higher critical density lines, e.g.,
between CO (1-0) and HCN (1-0), yield an approximate, first-order gauge of the
gas density [e.g. 119, 28, 177, 27]. The ngVLA would enable Milky Way science
across a broad spectrum of galaxies, at or below the cloud scale in both CO
and molecular gas tracers. The enormous sensitivity gain with ngVLA will open
up direct routine mapping of N2H+, the most robust tracer of dense star-
forming gas in the Milky Way, across entire galaxies, with high spatial
resolution for a diverse galaxy population. Lines such as HCN and HCO+ will be
mapped in fainter galaxies, including dwarfs and more distant galaxies
galaxies with more extreme starburst conditions. These direct constraints of
multiple dense gas tracers would provide a statistical view towards
understanding the exact role gas density plays in regulating star formation.

Stellar feedback is expected to contribute to preventing gas within a GMC from
undergoing gravitational collapse [e.g. 394, 311], setting the efficiency of
gas to form stars. Early feedback from both stellar winds and ionizing
radiation is crucial in setting the local conditions to maximize the impact of
supernovae events [281], and suppress the accretion of fresh gas soon after
the cluster has formed [121]. Recent studies have shown how supernova feedback
plays a critical role in shaping the structure of the ISM by driving large
$>$100 pc superbubbles into the molecular gas [400]. Without higher angular
resolution views of the molecular gas, we are unable to directly constrain the
point at which pre-supernova feedback processes dominate the early evolution
of these bubbles (Figure 22), or the efficiency with which these processes
couple with the gas [98], critical parameters with strong metallicity
dependencies when simulating star formation in galaxies. ngVLA observations
combined with new JWST data constraining the young stellar populations can
place direct quantitative constraints on these processes, even for the
earliest embedded phases of the cosmic matter cycle.

Figure 22: A superbubble in molecular gas in NGC 1566. The molecular ring
(blue-green colorscale, dashed line) traced in CO(2-1) is driven by the
stellar feedback processes from young star clusters (red-yellow colorscale).
By modeling the energetics of this system, it is possible to determine that
the bubble expansion must be driven by supernova explosions, as might be
expected for such large $>$100pc diameter features. Direct calculation of the
energetics due to stellar winds and radiation require a sharper view to
identify the smaller (earlier) bubbles within nearby galaxies. Figure adapted
from [400].

###  2.24 Wide-band polarization AGN studies, covering scales from parsecs to
kiloparsecs, measuring extreme Faraday rotation and complex Faraday effects

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Jack Livingston Institution: Max Planck Institute for Radio Astronomy
(MPIfR) Email<EMAIL_ADDRESS>
Co-authors: Yuri Kovalev (MPIfR) and Andrei Lobanov (MPIfR)

ngVLA Background: The ngVLA is a ground breaking extension to the existing VLA
and will provide may new opportunities in Active Galactic Nuclei (AGN)
research, in particular advancements in the study of the polarization of AGN.
The ngVLA will have 244 antennas with diameters of 18 meters, with 19
supplemental 6 meter diameter antennas to cover the sampling of short
baselines. These antennas will have six band receivers that span a total
coverage of 1.2 – 116 GHz. The longest baselines of the ngVLA array will be at
par with the current VLBA, while providing high-fidelity and high surface
brightness sensitivity and more complete UV-coverage than the VLBA. The
resolution of the maximum baselines at 93 GHz will be $\sim 0.08$ mas; for a
nearby AGN such as 3C120 ($z\sim 0.033$) with a central mass of $\sim 6\times
10^{7}M_{\odot}$, this means we could probe to around 10000 times the
Schwarzschild radius of the center engine.
Science Case for AGN Polarization Studies: The central engines of AGN generate
extreme luminosities, and are believed to be powered by supermassive black-
holes (SMBHs). These SMBH should have strong magnetic fields and high electron
densities in, and above, the inner regions of their accretion disks which play
a major role in the formation of relativistic jets that are visible at radio
frequencies. Magnetic fields of up to $B_{\mathrm{jet}}\sim 1$ G are typically
found near the base of the jet at around $r\sim 1$ pc [231, 306]. We can
measure these magnetic fields via the Faraday rotation of the polarization
angle of synchrotron radiation. Faraday rotation near the base of the jet,
assuming a mean electron volume density of $10^{2}\sim
10^{4}\,\mathrm{cm^{-3}}$, should be on the order of $10^{8}\sim
10^{10}\mathrm{rad\,m^{-2}}$. Using ALMA, [251] found a Faraday rotation value
for PKS 1830-211 of $10^{8}\mathrm{rad\,m^{-2}}$. Multiple Faraday rotating
media can be present along the line-of-sight as well as complex Faraday
effects like co-spatial Faraday rotation and polarized emission; RM-synthesis
using the ngVLA will allow us to measure extreme Faraday rotation and other
complex Faraday effects.
RM Synthesis with the ngVLA: With polarization data over a large range of
frequencies, the distribution of Faraday rotation along a sight-line can be
associated with the complex polarized surface brightness of emission, via RM-
synthesis [50], bypassing extreme Faraday depolarization and the n–$\pi$
ambiguity. This process can disentangle multiple Faraday rotating regions and
probe for the presence of other complex Faraday effects. The accuracy and
precision of RM-synthesis are highly dependent on the properties of observed
frequencies [50, 87]. With the capabilities of the ngVLA, we are able to
resolve Faraday rotation structures to
$\frac{69}{\mathrm{SNR}}\mathrm{rad\,m^{-2}}$ for the lowest frequency band
and probe Faraday rotation up to $\sim 10^{9}\mathrm{rad\,m^{-2}}$ with the
$70-116$ GHz band (channel width of $2$ MHz). With the current VLA, for the
VLA Q band ($40-50$ GHz, channel width of $2$ MHz), we are limited to $\sim
10^{8}\mathrm{rad\,m^{-2}}$. Unlike the VLBA, the superior sensitivity and
frequency coverage of the ngVLA mean we are not limited by extreme Faraday
rotation depolarization and the n–$\pi$ ambiguity, and the ngVLA will not be
as limited by opacity and resolution based effects which normally affect our
ability to observe extreme Faraday rotation with the VLA.

###  2.25 Astrophysical searches for physics beyond the Standard Model

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Andrei Lobanov Institution: Max-Planck-Institut für Radioastronomie,
Bonn Email<EMAIL_ADDRESS>
Co-authors:

Name: Aritra Basu (TLS Tautenburg), Simona Vegetti (MPA)

Finding experimental evidence for physics beyond the standard model (SM) of
particle physics is one of the pinnacles of the present day physical research.
A number of the SM extensions feature a new class of ultralight, weakly
interacting sub-eV particles (WISP), including axions, axion-like particles,
dark photons, millicharged particles, and chamaeleons [see 101, for a review],
most of which are also strong candidates for the dark matter and dark energy
"carrier" particles.

The ngVLA frequency range of 1.4–116 GHz will allow probing the existence of
such particles via different avenues, including signals from dark matter
conversion [305], astrophysical generation [108, 20], propagation through
space [234], and other specific effects such as birefringence [70, 19].

The impact of ngVLA in this areas can be exemplified by its potential for
detection of dark photons through their kinetic mixing with ordinary photons
which should result in oscillations between the normal photon state ($\gamma$)
and a "dark" or "hidden" photon state ($\gamma_{\mathrm{s}}$) in which the
photons acquire a non-zero mass and propagate along time-like geodesics but do
not interact with any normal matter [276, 157]. The properties of such dark
photon can be completely described by its mass $m_{\gamma_{\mathrm{s}}}$ and
the kinetic mixing parameter, or mixing angle, $\chi$ ($\ll 1$) describing the
probability of the oscillation. These oscillations will induce a specific
pattern [172] in broadband spectra of astrophysical objects, which can be used
for detecting dark photon and characterising their mass and kinetic mixing
parameter [234].

Figure 23: Expected sensitivity of ngVLA observations for detecting the dark
photon oscillations compared to the respective sensitivities of other
instruments. The sensitivity is calculated assuming observations of 100 AGN,
10 SNR, and 100 measurements of radio emission from outer planets. Dark photon
searches with the ngVLA would cover over ten orders of magnitude in the
particle mass in which they would provide unique constraints on the dark
photon coupling, $\chi$ to the ordinary photons. Calculations and figure are
adapted from [234].

The expected sensitivity of the ngVLA for detecting dark photon oscillations
is shown in Fig. 23 which compares the results that could be obtained by
combining together observations of 100 typical AGN, or 10 Galactic, or 100
measurements of the outer Solar system planets performed at different
separations from the Earth.

One can see that such measurements will cover $>10$ orders of magnitude of the
particle mass and will probe well into the couplings of $\chi\leq 0.01$ in
which dark photons can constitute the dark matter [133]. In all of this mass
range, the ngVLA would be one of the most competitive instruments, often
delivering the most sensitive measurements.

Figure 24: Expected sensitivity of the ngVLA to differential birefringence
from ultra-light ALPs.

Ultra-light axion-like particles: Light boson degrees of freedom, as for
example, axions and axion-like particles represent a potential and well-
motivated candidate for dark matter. One of the most interesting aspects of
this type of dark matter is that of achromatic birefringence [145, 64]. When
electromagnetic radiation propagates through axionic fields, a tiny
interaction ($g_{a\gamma}$) between each other leads to different phase
velocities of the right- and left-handed polarisation states of the photons.
As a consequence, the plane of a linearly polarized light undergoes rotation
by $\Delta\theta_{a}$ which directly depends on the properties of the axionic
field. Observations of astrophysical and cosmological sources that emit
polarised light have been used to detect this effect and set constraints on
the properties of the axionic field [170, 114]. However, due to instrumental
and calibration related offsets, and often insufficient knowledge of the
intrinsic polarization of the source, $\Delta\theta_{a}$ is difficult to
measure. Strong gravitational lensing provides a measure of differential
birefringence between the multiple images of the same lensed source. Hence it
represents a unique opportunity to constrain the axion field in a way that
alleviates all astrophysical and instrumental systematics [19].

The high angular resolution, and sensitivity of the ngVLA will play a pivotal
role in advancing the search for ultra-light axionic fields with mass
$\lesssim 10^{-17}$ eV. In particular, broadband polarimetry will allow for
searching for differential birefringence signals from strong gravitational
lenses that produce multiple images. As can be seen from Fig. 24, the expected
parameter space a sample of gravitational lenses monitored for 5 years with
the ngVLA will pessimistically probe is already comparable to the cosmic
variance limit of birefringence measurements via the polarized CMB [106].

###  2.26 Physics and astrometry with photon rings

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ Galaxies
and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Andrei Lobanov Institution: Max-Planck-Institut für Radioastronomie,
Bonn Email<EMAIL_ADDRESS>
Co-authors:

Name: Eduardo Ros (MPIfR), Anton Zensus (MPIfR)

The remarkable success of the Event Horizon Telescope (EHT) using very long
baseline interferometry (VLBI) at 230 GHz (wavelength $\lambda=1.3$ mm) for
imaging ring like structures in Sgr A⋆ [102] and M87 [103] has opened up new
perspectives in detecting and characterising relativistic photon ring
structures in AGN and using these measurements for addressing a range of
fundamental questions [183].

Studies of photon rings are one of the key science drivers for upgrading the
EHT, including the plans next generation EHT (ngEHT, [183]), and potentially
also for future space VLBI missions [185]. These approaches largely rely on
increasing the instrumental resolution, $\theta_{\mathrm{inst}}\approx
1.22\lambda_{\mathrm{obs}}/B_{\mathrm{max}}$, by either shortening the
observing wavelength, $\lambda_{\mathrm{obs}}$, or increasing the maximum
baseline, $B_{\mathrm{max}}$, of interferometric measurements.

However, the effective resolution, $\theta_{\mathrm{im}}$, of interferometric
observations also depends on signal-to-noise ratio of visibility measurements
[232, 230] and, respectively, on the dynamic range, $D_{\mathrm{im}}$, of
images reconstructed from these visibilities, so that
$\theta_{\mathrm{im}}\approx\theta_{\mathrm{inst}}/\sqrt{\ln
3\,D_{\mathrm{im}}}$ . This effect, commonly referred to as "superresolution"
of CLEAN deconvolution of interferometric images, has now been fully exploited
in a range of new imaging algorithms and successfully demonstrated by
recovering a ring like structure in M87 from VLBI observations at 86 GHz
($\lambda=3.6$ mm) [238].

Observations with the ngVLA are expected to provide an unprecedented
improvement of visibility and image noise [347], and at 93 GHz they should
reach down to $\sigma_{\mathrm{rms}}=4.5\,\mu\mathrm{Jy}$ on angular scales
below 100 $\mu$as within one hour of observing time. This should allow for
imaging the most compact emission at dynamic ranges $D_{\mathrm{im}}\gtrsim
200\,000\,[S/1\,\mathrm{Jy}]$, which should result in considerable
improvements of both effective resolution and accuracy of positional location
of compact features such as photon rings.

The resulting positional accuracy would approach microarcsecond level, similar
to that expected from frequency phase transfer mode of VLBI at 86 GHz [89],
which would provide a firm foundation for performing state of the art
astrometric measurements of annual parallaxes at distances up to $\approx 100$
kpc, galactic proper motions up to distances of $\sim 10$ Mpc, and secular
parallaxes at distances up to $\sim 100$ Mpc.

Prospects of ngVLA observations to be used for photon ring detection and
characterisation are presented in Fig. 25 which outlines dynamic ranges
required to distinguish between a ring of finite thickness and other types of
compact structure. The figure demonstrates that ngVLA would provide excellent
capabilities for imaging both M87 and Sgr A⋆ at 116 GHz (overcoming the
$0.76\times 0.40$ mas scatter broadening expected at this frequency [168]) and
may also enable robust photon ring characterisation in a good subset of
objects considered for ngEHT observations [310]. The latter objects will
particularly benefit from operating the ngVLA (with $B_{\mathrm{max}}=8680$
km) in the VLBI mode, with external antennas on transcontinental baselines
including potential new locations in Germany and Europe, increasing the
maximum baseline length to over 11,000 km. In this capacity, ngVLA will have
an exceptionally strong impact on studies of photon rings.

Figure 25: Estimated capability for detecting and characterising photon rings
in M87, Sgr A⋆, and selected AGN from the ETHER sample [310], compared for the
existing EHT (purple stars; [103, 102]) and GMVA (grey star; [238])
observations and at dynamic ranges, $D_{\mathrm{im}}$, projected for the ngEHT
[309] (blue stars) at 230 GHz and for 116 GHz observations with the ngVLA
[348] (orange stars) and ngVLA operating in VLBI mode (red stars). For all
arrays, the maximum sampled spatial frequency,
$B_{\mathrm{max}}/\lambda_{\mathrm{obs}}$, is normalized by the spatial
frequency, $q_{\mathrm{null,1}}$, of the first null in the visibility response
of a thin ring. Scatter broadening [168] is taken into account in the ngVLA
estimates for Sgr A⋆. The four curves show the minimum dynamic ranges which
would allow distinguishing a ring with finite thickness from the respective
types of structure observed on the most compact scale accessible for a given
array. The remarkable sensitivity improvements of the ngVLA should make it
highly competitive for detecting and characterising potential photon ring
structures in AGN, particularly when operating in VLBI mode with external
antennas on transcontinental baselines.

###  2.27 Scattering mitigation of the Galactic Center using ngVLA

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ ✓ The Galaxy and the Local Group $\square$ ✓
Galaxies and AGN $\square$ Cosmology, Large Scale Structure and Early Universe
$\square$ ✓ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Hendrik Müller & Alejandro Mus999both authors have and will equally
contribute Institution: MPIfR, Bonn & Univ. de València, Valéncia Email:
<EMAIL_ADDRESS><EMAIL_ADDRESS>
Co-authors: Andrei Lobanov, Eduardo Ros & Guang-Yao Zhao

Motivation – The Galactic Center (GC) represents an invaluable astrophysical
laboratory, hosting a diverse assemblage of celestial remnants, including
supernova remnants, pulsars, magnetars, and radio transients, among other
sources [152]. A focal point of paramount scientific interest is the super-
massive black hole, Sagittarius A* (SgrA*) [e.g. 137, 102]. However, the GC is
susceptible to the perturbing influence of the interstellar medium (ISM),
giving rise to refractive and diffractive scattering phenomena that
significantly perturb visibility measurements, particularly at frequencies
below approximately 120 GHz [184, 168].

The forthcoming capabilities of the ngVLA hold the promise of enabling
observations of SgrA* even at lower frequencies than attainable with the Event
Horizon Telescope (EHT), thereby expanding the dynamic range of observations.
Nonetheless, an effective strategy for the mitigation of scattering effects is
imperative.

To address this challenge, the German research team is poised to develop
innovative mathematical tools aimed at ameliorating the influence of
scattering, thus affording astronomers the ability to attain high-quality
imaging results. The degree of scattering is intrinsically linked to the
square of the observation wavelength ($\lambda^{2}$), whereby higher
frequencies experience proportionally reduced scattering effects. The
extensive frequency bandwidth envisioned for the ngVLA is poised to capitalize
on this wavelength-dependent characteristic, albeit with stochastic
perturbations affecting visibilities unevenly. The research team advocates for
the introduction of a novel mathematical function, termed a "regularizer,"
which leverages high-frequency visibilities to counterbalance the adverse
impacts of scattering upon their low-frequency counterparts.

Methodology – To tackle the problem of the stochastic scattering noise
affecting the visibilities, we propose a mathematical function that will use
the full wide bandwidth to mitigate the corruptions. A first regularizer was
developed in [182, 186, 169]. This regularizer will be included in a forward-
modeling approach. Due to a number of degeneracies between the intrinsic
resolution and variability of SgrA*, and the properties of the scattering
screen itself, a multimodal and multiobjective strategy is needed as recently
developed in [265, 267]. In this way, even a dynamical multifrequency
reconstruction of the screen will be able to be done.

Science cases – Once this method is successfully implemented, the advantages
for the astronomical community will be profound, with implications for various
aspects of astrophysical research:

  * •

Constrain the Scattering Screen Mapping: The ability to constrain the
scattering screen during one long observation holds significant scientific
value. This technique would enable astronomers to investigate the temporal
evolution of interstellar scattering, to better deal with the turbulence and
structures in the interstellar medium. Exploiting the wide bandwidth on high
frequencies to constrain and mitigate the variability on lower frequencies,
even doing so in a dynamic way, can provide a better calibration techniques
that will improve our understanding of celestial objects affected by these
screens, such as pulsars and distant galaxies.

  * •

Improved Transient Detection and Astrometry: The mitigation of scattering
effects is essential for the precise detection and characterization of
transient astrophysical phenomena, including Fast Radio Bursts (FRBs), gamma-
ray bursts, and transients. In particular, recent “super-slow” pulsars have
been observed [59, 24], but none near the GC. By reducing scattering-induced
distortions and signal loss, we can obtain more accurate information about
these events, even increasing the chances to find exotic sources such as
‘super-slow” neutron stars.

  * •

Observing SgrA* at Lower Frequencies and Frequency-Dependent Ring Shadow
Analysis: Lower-frequency observations of SgrA* offer the potential for more
detailed investigations of the black hole’s immediate vicinity. It can
facilitate the study of the shadow cast by SgrA*, the dynamics of hotspots in
its vicinity, and determine whether the appearance of the ring shadow is
frequency-dependent. This analysis is crucial for testing General Relativity,
validating or refining models of black hole accretion, and assessing potential
deviations from its predictions. In essence, lower-frequency observations not
only probe a larger area in the emission region but also provide opportunities
to advance our understanding of the fundamental physics governing black hole
dynamics and gravity in strong gravitational fields.

In summary, the successful implementation of our method holds great promise
for advancing the frontiers of astrophysical research, enabling scientists to
probe the underlying physics of scattering screens, transient phenomena, and
the fundamental nature of black holes and gravity in a more nuanced and
precise manner. These scientific advancements would deepen our understanding
of the universe and its intricate mechanisms.

###  2.28 The Cosmic Density and Excitation of Cold Molecular Gas

Thematic Areas: $\square$ Stellar Astrophysics $\square$ Solar System,
Planetary Systems and Habitability $\square$ Circuit of Cosmic Matter (incl.
star formation) $\square$ The Galaxy and the Local Group $\square$ ✓ Galaxies
and AGN $\square$ ✓ Cosmology, Large Scale Structure and Early Universe
$\square$ Extreme conditions in the cosmos, fundamental astrophysics

Principal Author:

Name: Dominik A. Riechers Institution: Universität zu Köln Email:
<EMAIL_ADDRESS>
Co-authors: Fabian Walter (MPIA), Leindert Boogaard (MPIA), Ryan Keenan
(MPIA), Pierre Cox (IAP)

Substantial efforts including multiple large programs on the Atacama Large