[1,2,3]Valentina D’Odorico

[1]INAF - Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34143
Trieste, Italy 2]Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126
Pisa, Italy 3]IFPU - Institute for Fundamental Physics of the Universe, Via
Beirut 2, 34014 Trieste, Italy 4]School of Physics and Astronomy, University
of Nottingham, University Park, Nottingham NG7 2RD, UK 5]Niels Borh Institute,
University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen, Denmark 6]European
Southern Observatory, Karl-Schwarzschild Str. 2, D-85748 Garching bei München,
Germany 7]Department of Astronomy, University of Geneva, Chemin Pegasi 51,
1290 Versoix, Switzerland 8]Department of Physics and Astronomy, Uppsala
University, Box 516, SE-751 20 Uppsala, Sweden 9]Swedish Collegium for
Advanced Study, Linneanum, Thunbergsvägen 2, SE-752 38 Uppsala, Sweden 10]INAF
- Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131
Napoli, Italy 11]Purple Mountain Observatory, Chinese Academy of Sciences, 10
Yuanhua Road, Nanjing 210023, People’s Republic of China 12]Cavendish
Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE,
UK 13]Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze,
Via G. Sansone 1, Italy 14]INAF - Osservatorio Astrofisico di Arcetri, Largo
E. Fermi 5, I-50125 Firenze, Italy 15]Institut für Physik und Astronomie,
Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam,
Germany 16]Leibniz-Institut für Astrophysik (AIP), An der Sternwarte 16, 14482
Potsdam, Germany 17]GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place
Jules Janssen, 92190 Meudon, France 18]Dipartimento di Fisica G. Occhialini,
Università degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126
Milano, Italy 19]Institut d’Astrophysique de Paris, UMR 7095, CNRS-SU, 98bis
bd Arago, 75014 Paris, France 20]Instituto de Astrofísica e Ciências do Espaço
- Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762
Porto, Portugal 21]Sub-department of Astrophysics, University of Oxford, Denys
Wilkinson Building, Keble Road, Oxford OX1 3RH, UK 22]INAF - Osservatorio di
Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129
Bologna, Italy 23]INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46,
23807 Merate, Italy

# Galaxy Formation and Symbiotic Evolution with the Inter-Galactic Medium in
the Age of ELT-ANDES

<EMAIL_ADDRESS>James S. Bolton  Lise Christensen  Annalisa De Cia
Erik Zackrisson  Aron Kordt  Luca Izzo  Jiangtao Li  Roberto Maiolino
Alessandro Marconi  Philipp Richter  Andrea Saccardi  Stefania Salvadori
Irene Vanni  Chiara Feruglio  Michele Fumagalli  Johan P. U. Fynbo  Pasquier
Noterdaeme  Polychronis Papaderos  Céline Péroux  Aprajita Verma  Paolo Di
Marcantonio  Livia Origlia  Alessio Zanutta  *  [  [  [  [  [  [  [  [  [  [
[  [  [  [  [  [  [  [  [  [  [  [

###### Abstract

High-resolution absorption spectroscopy toward bright background sources has
had a paramount role in understanding early galaxy formation, the evolution of
the intergalactic medium and the reionisation of the Universe. However, these
studies are now approaching the boundaries of what can be achieved at ground-
based 8-10m class telescopes. The identification of primeval systems at the
highest redshifts, within the reionisation epoch and even into the dark ages,
and of the products of the first generation of stars and the chemical
enrichment of the early Universe, requires observing very faint targets with a
signal-to-noise ratio high enough to detect very faint spectral signatures. In
this paper, we describe the giant leap forward that will be enabled by ANDES,
the high-resolution spectrograph for the ELT, in these key science fields,
together with a brief, non-exhaustive overview of other extragalactic research
topics that will be pursued by this instrument, and its synergistic use with
other facilities that will become available in the early 2030s.

###### keywords:

Galaxy formation and evolution, Intergalactic medium, Circumgalactic medium,
Interstellar medium, High-resolution spectroscopy, ANDES

##  1 Introduction

After an initial hot, dense phase, the Universe expanded and cooled enough to
enable protons and electrons to form the first hydrogen atoms. The epoch that
followed is known as the “Dark Ages”. The first generation of stars and
galaxies formed from gas collapse approximately $\sim 100-200$ Myrs after the
Big Bang and started flooding its surroundings with highly energetic
ultraviolet (UV) radiation. These UV photons (with energies $E>13.6\rm\,eV$)
brought the hydrogen atoms in the intergalactic medium (IGM) back into a
highly ionised state. This important physical transition, which impacted
almost every baryon in the Universe, is known as the Epoch of Reionisation
(EoR). The same first stars enabled also the production of the first chemical
elements heavier than hydrogen and helium (dubbed “metals”) beyond those
produced by Big Bang nucleosynthesis, thereby opening the stage for the
chemical enrichment of the early Universe.

At present, the ionising photon budget for reionisation is thought to have
been dominated by radiation emitted by the first generations of star-forming
galaxies [e.g. 85, 107], although it is still debated whether accreting black
holes may also have provided a substantial contribution [e.g. 66, 50]. The
fraction of ionising photons that eventually escaped from these sources into
the IGM is also uncertain. Despite the recent discovery by JWST of unexpected
excesses of both bright galaxies [e.g., 108] and faint active galactic nuclei
[AGN, e.g. 70, 105, 104] at $z\gtrsim 7-10$, a complete account of all
ionising sources in the early Universe remains very challenging.

A less direct but powerful, complementary approach is to focus on the
signature that the first galaxies will leave on their surrounding gaseous
environments. Intergalactic gas in the early Universe represents the reservoir
for the formation and sustenance of gravitationally bound luminous sources,
but is also sensitive to the escape of ionising radiation and heavy elements
from them, even from those that are too faint to be directly imaged.
Reionisation-era sources will therefore have a measurable impact on the
ionisation state, temperature and metallicity of the IGM [e.g., 9].

The investigation of this interplay between galaxies and the IGM through
cosmic history is central to the understanding of galaxy formation and
evolution. Most of the interaction between galaxies and the IGM happens on
scales of the order of 1 physical Mpc or less, corresponding to $\lesssim 2$
arcmin at $z\sim 2.5$ ($\lesssim 3$ arcmin at $z\sim 6$), and this region has
been aptly termed the circum-galactic medium (CGM).

Spectroscopic observations focused on absorption lines are the method of
choice to study the diffuse gas outside galaxies but also the interstellar
medium (ISM). Multiple, complementary probes are available: neutral hydrogen
Lyman-series lines (in particular, H i Lyman-$\alpha$ and Lyman-$\beta$),
transitions of heavier elements (e.g. C, O, Si, Mg, Al and Fe, to cite the
most common ones), as well as molecular transitions (due to e.g. H2, HD, CO).
Accurate chemical abundances and physical conditions can be derived from
absorption spectroscopy up to the highest redshifts probed by bright
background sources, at variance with other high-$z$ galaxy measurements
derived from emission line diagnostics.

In this paper, we highlight the outstanding questions related with galaxy
formation and evolution that can be answered with the ArmazoNes high
Dispersion Echelle Spectrograph (ANDES111https://andes.inaf.it/) for the
Extremely Large Telescope (ELT), mainly through the properties of the ISM, CGM
and IGM studied in absorption in the spectra of bright background sources.

High-resolution spectroscopy has been key to investigate the properties of the
diffuse gas in terms of spatial distribution, physical conditions and chemical
abundances at intermediate redshift, $z\sim 2-4$, e.g. determining the
temperature of the IGM and the intensity of the UV background impinging on it
[e.g. 56, and references therein], assessing the existence of large amount of
metals in the gas surrounding galaxies [e.g. 153] but also their presence in
the shallow intergalactic gas [e.g. 39]. ANDES, thanks to its wavelength
extension to the near-IR ($0.4-1.8$ $\mu$m, goal $0.35-2.4$ $\mu$m), its
spectral resolution ($R\sim 100,000$), and the unprecedented collecting area
of the ELT, will allow us for the first time to carry out very detailed
studies of the H i distribution, the metal content and chemical (relative)
abundances at the epoch of Reionisation (EoR). At the same time, we will be
able to significantly increase the sample of objects that can be studied at
high resolution in the intermediate redshift regime ($z\sim 2-4$) with
unprecedented sensitivity for the weak metal absorption lines, which is beyond
the reach of current 8-10m class telescopes.

The paper is structured as follows. Section 2 describes the science case which
deals with the characterisation of the epoch of Reionisation through the study
of the Lyman-$\alpha$ and Lyman-$\beta$ forests in the spectra of $z\gtrsim 6$
quasars and $\gamma$-ray bursts (GRBs). Section 3 highlights how ANDES will
unveil the chemical enrichment of the early universe and the nature of the
first stars. Section 4 is dedicated to the extragalactic transient phenomena
whose investigation will be allowed by ANDES. In Section 5, we briefly
describe a few additional science cases hinting at the potentiality of ANDES
in this field. In Sections 6 and 7 we report the benefits of an extension to
the U band and of the addition of a mode with resolving power $R\simeq
50,000$, respectively. Finally, in Section 8 we highlight the synergies
between ANDES at the ELT and other surveys or facilities.

##  2 Investigating the reionisation epoch with absorption spectroscopy

In the forthcoming decade, the detection of the redshifted H i $21$-cm
emission and/or absorption from the neutral IGM at redshift $z>6$ is a key
goal of radio interferometric arrays such as the Square Kilometre Array [SKA,
e.g., 94]. This approach holds enormous promise for characterising the nature
of the first stars and galaxies across a broad range of time that stretches
from reionisation at $z\sim 6$ to the end of the cosmic dark ages at $z\sim
20$–$30$. However, the $21$-cm observations are subject to challenging
foregrounds, and during reionisation the $21$-cm power spectrum and/or
tomographic maps will be primarily sensitive to the distribution of the
neutral hydrogen gas on the $>10$ comoving Mpc scales typical of ionised
regions [e.g., 102]. Furthermore, the results of $21$-cm experiments will
require independent verification by other methods.

Figure 1: The upper panel shows a segment of the Lyman-$\alpha$ (black curve)
and coeval Lyman-$\beta$ (blue curve) forests in the spectrum of a $z=6.1$
quasar, drawn from a cosmological hydrodynamical simulation coupled with
radiative transfer [124]. Note that the Lyman-$\beta$ forest flux has been
offset by $+0.1$ for presentation purposes, and it also includes foreground
Lyman-$\alpha$ forest absorption at
$z^{\prime}=\lambda_{\beta}(1+z)/\lambda_{\alpha}$, where
$\lambda_{\alpha}=1215.67\rm\,\AA$ and $\lambda_{\beta}=1025.72\rm\,\AA$. The
lower three panels show simulated observations with an exposure time of 4
hours assuming a magnitude $m_{\rm AB}=20$ for the target quasar, obtained
with (from top to bottom): VLT-XSHOOTER at $R\sim 10,900$ [see e.g., 41] with
S/N$\sim 12$, ELT-ANDES for the baseline resolution of $R=100\,000$, reaching
S/N $\sim 77$ and for the lower resolution mode with $R=50\,000$ with S/N
$\sim 113$. All signal-to-noise ratios are per 10 km s-1 spectral element.
Figure 2: O$\,\rm\scriptstyle I\ $absorbers at $z\sim 6.7$ in the simulated
spectrum of a quasar with magnitude $m_{\rm AB}=20$. As for Figure 1, the
lower three panels show simulated observations with VLT-XSHOOTER at $R\sim
10,900$ and ELT-ANDES for the baseline resolution of $R=100,000$ and a
possible lower resolution mode with $R=50,000$.

An alternative, well-established and highly complementary approach, that is
also sensitive to gas on much smaller scales ($<1$ comoving Mpc, see Figure
1), is absorption line spectroscopy. Indeed, the majority of our current
observational understanding of the later stages of Reionisation has been
derived from studying the IGM using optical and near-IR spectroscopy. For
example, variations in the Lyman-$\alpha$ opacity observed in the spectra of
bright quasars at $z>5.5$ are consistent with the final stages of reionisation
ending as late as $z\sim 5.3$ [97, 21]. The rather small ionising photon mean
free path at $z\sim 6$ inferred from the Lyman continuum opacity measured in
high redshift quasar spectra points to a similar conclusion [11, 184]. The
widths of Lyman-$\alpha$ transmission spikes at $z>5$ have been used to
measure the temperature of the IGM [55], a quantity that is sensitive to the
spectral shape of the ionising radiation from the first galaxies. Furthermore,
the three-dimensional relationship between Lyman-$\alpha$ transmission spikes,
metal lines, and high redshift galaxies has yielded new insights into the
escape of enriched gas [37] and the escape fraction of ionising radiation into
the IGM [111, 88]. Dark gaps and pixels in the Lyman-$\alpha$ forest can be
used to measure (or place limits on) the IGM neutral fraction at $5<z<7$ [183,
86]. Finally, Lyman-$\alpha$ damping wings in the spectra of quasars [165],
gamma-ray bursts [100] and (now, with JWST) star forming galaxies at
$z=6$–$12$ [154, 74] are sensitive to intergalactic and circumgalactic neutral
hydrogen deep into the reionisation era.

The enormous advantage that ANDES brings to this endeavour is the dramatic
improvement in spectral resolution and sensitivity that is attainable with a
high resolution spectrograph on the ELT. At present, only modest samples of
the brightest ($m_{\rm AB}\lesssim 20$ mag) among the quasars at $z\gtrsim 6$
can be gathered with high enough sensitivity to carry out reliable studies at
the EoR and only at intermediate resolution ($R\lesssim 10,000$) [e.g. 41].
ANDES will allow us to increase these samples from tens to hundreds of
background sources (moving the magnitude threshold to $m_{\rm AB}\sim 21$ mag)
and study both Lyman-series transmission features and metal absorption lines
with a sensitivity and a resolution ($R\sim 100,000$) never obtained before.
Should a $R=50,000$ mode also be implemented in ANDES, then we could have
access to the majority of the quasars at $z\gtrsim 6$ known today [48, for an
updated catalogue] with $m_{\rm AB}\sim 21-23$ mag, that cannot be used today
for the proposed studies, therefore outstandingly improving the statistical
significance of constraints on key properties (e.g., the average H I
fraction). This will allow ANDES to constrain the patchiness of the
reionisation process, the properties of the ultraviolet background (UVB)
radiation, and the thermal state and chemical enrichment of the IGM to high
precision.

Figure 1 illustrates a mock line of sight through the IGM at $z\sim 6$ drawn
from a cosmological hydrodynamical simulation that has been coupled with
radiative transfer [124], and that assumes a late, inhomogeneous reionisation
that completes at $z\sim 5.7$. The increasingly neutral IGM at $z\geq 5.5$
produces largely saturated Lyman-$\alpha$ forest absorption (i.e. the Gunn-
Peterson trough, Gunn and Peterson 69) blueward of the Lyman-$\alpha$ of the
background source. However, underdense, highly ionised regions of the IGM may
nevertheless produce narrow transmission features in this part of the
spectrum. The smaller absorption cross-section of the Lyman-$\beta$ transition
furthermore allows transmission from ionised regions that otherwise remain
saturated in Lyman-$\alpha$ absorption. For example, several regions with
saturated Lyman-$\alpha$ absorption (black curves) are highlighted with
horizontal red arrows in the upper panel of Figure 1. There is a small $\sim
10$ comoving Mpc island of fully neutral hydrogen ($x_{\rm HI}=1$) at $z\sim
5.83$ that exhibits coeval saturated Lyman-$\beta$ absorption (blue curves).
In contrast, the region of saturated Lyman-$\alpha$ absorption between
$z=5.855$–$5.875$ is highly ionised ($x_{\rm HI}\sim 10^{-4}$), as indicated
by the coeval Ly$\beta$ transmission peaks that extend over scales of
$\lesssim 1$ comoving Mpc. The simulated observations in the lower three
panels highlight the unique capabilities of ANDES for probing Lyman series
absorption at the tail-end of the reionisation era. The narrow features in the
forest are easily detected by ELT-ANDES, but are not accessible with VLT-
XSHOOTER. By detecting such features, ANDES will provide constraints on the
patchiness of the reionisation at scales that are highly complementary to
those probed by SKA. Furthermore, the intrinsic (thermal) widths of these
features (where $b_{\rm Ly\alpha}\sim
12.9\rm\,km\,s^{-1}(T/10^{4}\rm\,K)^{1/2}$, cf. $\rm FWHM\sim
6\rm\,km\,s^{-1}$) will be resolved by ANDES even at $R\sim 50,000$ (if the
medium resolution mode is implemented), therefore enabling the temperature of
the IGM to be measured [19]. The locations of these transmission spikes may
also be correlated with galaxies detected in emission with IFU observations or
slitless spectroscopy [e.g. with JWST, see 88], yielding insight into the
relationship between the IGM and the sources of ionising photons.

Above redshift $z\sim 6.5$, the Lyman forest becomes less and less informative
for what concerns the amount and the distribution on neutral hydrogen, due to
the almost complete absorption of the quasar flux. However, high-resolution,
$z\gtrsim 6.5$ quasar spectra could still be useful for the study of the
topology of the reionisation process thanks to the presence of metal
absorption lines in the region of the spectrum not affected by H i absorption.
Some of these metal absorption lines could trace the mostly neutral IGM, if
this is polluted to metallicities $\log(Z/Z_{\odot})\sim 3.5-2.5$. Such a
minimal level of metal pollution is expected at the EoR if stars in galaxies
(and not AGN) are the main ionizing sources. The O$\,\rm\scriptstyle I\ $line
at $1302$ Å is particularly promising: the O and H first ionization potentials
are almost identical, and O$\,\rm\scriptstyle I\ $should be in very tight
charge exchange equilibrium with H i. The Si$\,\rm\scriptstyle II\ $$1260$ Å
transition might also be observable. At high redshift, overdense regions are
the first to be polluted to high metallicity but the last to remain
permanently ionised, as a result of the short recombination times. Such
regions should produce a fluctuating O$\,\rm\scriptstyle I\ $and
Si$\,\rm\scriptstyle II\ $forest, which, if observed, would indicate large
quantities of neutral hydrogen [116]. Furthermore, cosmological hydrodynamical
simulations with on-the-fly multi-frequency radiative transfer shows that,
concurrent with the development of the hydrogen reionisation process,
O$\,\rm\scriptstyle I\ $absorbers undergo a decrease in their footprint around
haloes and a decrease in the number of observed systems [42]. This decrease
occurs preferentially for absorbers that are farther from haloes (in the IGM
and CGM) and that have lower equivalent width (EW $\lesssim 0.05$ Å). State-
of-the-art observations at intermediate resolution ($R\lesssim 10,000$) of a
sample of $\sim 200$ quasars covering the redshift range $3.2<z<6.5$ show a
drop of the number density of O$\,\rm\scriptstyle I\ $absorbers with EW $\geq
0.05$ Å at $z\lesssim 5.7$ [10, Sebastian et al. subm.], evocative of the
evolution suggested by simulations due to the change in the ionisation state
of the CGM gas. However, present facilities do not allow to gather
statistically significant, complete samples of low equivalent width
O$\,\rm\scriptstyle I\ $absorption lines.

Only thanks to the sensitivity and resolving power of ANDES, it will be
possible to detect the weak metal lines due to, in particular,
O$\,\rm\scriptstyle I\ $$1302$, Si$\,\rm\scriptstyle II\ $$1260$ and also
C$\,\rm\scriptstyle II\ $$1334$ Å up to the highest redshifts probed by
quasars (and possibly GRBs, see Section 4). An example of the capability of
ANDES to reveal these tracers of the early chemical enrichment is illustrated
by the simulated spectrum of a $z\sim 7$ quasar in Figure 2. The model is
drawn from a cosmological hydrodynamical simulation of inhomogeneous
reionisation with a volume averaged neutral fraction of $\langle x_{\rm
HI}\rangle=0.38$ at $z=6.7$ [124]. Following Keating et al [89], a density
dependent metallicity of $[\rm O/H]=-2.65+1.3\log_{10}(\Delta/80)$ is assumed,
where $\Delta=\rho/\langle\rho\rangle$ is the gas density relative to the mean
and $[\rm O/H]_{\odot}=-3.31$ [5]. In this model, strong O$\,\rm\scriptstyle
I\ $absorbers typically arise from circumgalactic gas that remains well-
shielded from UV radiation (i.e. with a neutral hydrogen fraction $x_{\rm
HI}\sim 1$, see also Doughty and Finlator 42). The figure also shows how this
kind of science is challenging for current facilities (e.g. VLT-XSHOOTER).

##  3 The metal era: the onset of metal enrichment in the Early Universe

Metals in the ISM and CGM of galaxies have a crucial role in shaping the
formation and evolution of galaxies, stars, planets, and molecules. All
chemical elements heavier than beryllium that we observe today were formed in
diverse stellar processes throughout the history of the universe. The first
generation of stars (Pop III) were likely very massive and short-lived
[possibly between 10 and 1000 $M_{\odot}$, 75, 76, 96]. PopIII stars were born
from pristine gas and polluted their surroundings with the first metals, thus
starting the chemical enrichment of the early universe and marking the onset
of the “metal era”.

Simulations predict that the first stars appeared around $z\sim 20$–30 in the
so-called H2-cooling low-mass minihalos (e.g. Abel et al 1, Yoshida et al 181
and for a review Bromm 22, Klessen and Glover 91) and that they rapidly
enriched the ISM [e.g. 68, 174] along with the IGM [e.g. 151, 117, 84] with
their newly produced heavy elements. Observationally, direct detection of
PopIII stars is beyond the reach of present and currently foreseen
facilities222With the possible exception of gravitationally lensed, individual
PopIII stars [e.g. 173, 146], which are potentially detectable in imaging, but
for which it would be extremely hard, if not impossible, to establish whether
they actually are Pop III (e.g., by measuring their metallicity)., with JWST
having the potential to unveil emitting gas at low-metallicity at high-$z$
[156, 106]. A powerful way to investigate the early chemical enrichment is
instead to study the chemical signature of Pop III stars in gas throughout the
universe by comparing the gas (relative) abundances with the expected metal
yields of Pop III stars and related stellar populations (Sect. 3.1). ANDES
will be transformational in unveiling the early metal enrichment by studying
very-metal-poor absorbing systems tracing pristine gas at intermediate
redshifts (Sect. 3.2) as well as gas-rich galaxies or absorbing systems at
very high redshift towards quasars and GRBs (Sect. 3.3).

Figure 3: Chemical abundance ratios predicted for a gaseous environment
imprinted by a single zero-metallicity Pop III SN (faint SNe: green; core-
collapse SNe: blue; hypernovae: purple; PISN: red). The intensity of the
colour increases with probability. For comparison we show in grey the
predicted chemical abundances of an environment solely imprinted by normal Pop
II stars.

###  3.1 Expected metal yields of Pop III stars

The first Pop III SNe can explode with a variety of energies. Pop III stars
with $m_{*}=140-260\ M_{\odot}$ are predicted to end their life as Pair
Instability Supernovae [PISN, e.g. 72, 147] whose explosion energy increases
with the stellar mass ($E=10^{52-53}$ erg) and is able to completely destroy
the progenitor star. Conversely, Pop III stars in the mass range
$m_{*}=10-1000\ M_{\odot}$ can, independently of their mass, evolve as SNe
with different energies, ${\rm E=(0.3-10)\times 10^{51}}$ erg, thus releasing
in the surrounding gas different chemical elements [e.g., 73]. General
theoretical models for Pop III stars’ enrichment can be used to pinpoint the
chemical signature of these different Pop III SNe [133, 155]. In Fig. 3, we
report the diagnostic diagrams introduced by Vanni et al. (submitted), which
enable us to disentangle the chemical signature left by different Pop III SNe
in high-$z$ absorbers, where the H i column density can not be always
measured. It is clear from the figure that gaseous environments solely
imprinted by PISNe dwell in very specific regions of the diagrams. Thus, we
can potentially identify them by measuring the proposed abundance ratios.
Conversely, environments imprinted by less massive, $m_{*}=10-1000\
M_{\odot}$, Pop III SNe overlap for several chemical abundances and reside in
regions of the diagrams where we can also find the imprint of normal Pop II
SNe [see 155, for details]. This makes their identification more challenging.
We should also emphasize that while the yields of PISNe are extremely solid,
the predictions for less massive Pop III (and Pop II) SNe are more unsure
[e.g. see Sec. 2.2 of 168]. To minimize this issue in Fig. 3 we adopted the
most complete set of yields for Pop III SNe with $m_{*}=10-1000\ M_{\odot}$ by
Heger and Woosley [73], which accounts for different possible explosion
energies and stellar mixing efficiencies (see Sec. 5 and Fig. 10 by
Koutsouridou et al [96] for a comparison with other yields). For Pop II SNe,
furthermore, we show their IMF-integrated contribution for two different sets
of yields [176, 101].

###  3.2 Metal-poor gas at intermediate redshift

Finding and studying near-pristine gas can be an effective way to trace gas
enriched by the first generation of stars. Very-metal poor
[$Z<10^{-2}Z_{\odot}$ 26, 12] gas in absorbing systems such as Lyman Limit
systems (LLS) and Damped Lyman-$\alpha$ systems (DLAs)333These absorption
systems are defined by their H i column density. LLS have
$17.3\leq\log(N/\rm{cm}^{-2})<20.3$, while DLAs are characterised by
$\log(N/\rm{cm}^{-2})\geq 20.3$ has been observed at $z\sim 2-4$ [e.g. 52,
168, 169, 130]. The existence of such systems at intermediate redshifts
implies that that the mixing of metals in gas is still incomplete 10 Gyr after
the Big Bang, so it is promising to search for signatures of the first
generation of stars in gas even at cosmic noon. In some cases gas that had
possibly been enriched by a single generation of stars was observed [25, 29,
27]. Recently, Saccardi et al [131] observed C-enhanced very metal-poor
absorbing systems, which represent the high-$z$ analogue of Carbon enhanced
metal poor (CEMP-no) stars observed in the Local Group. These peculiar
absorbers are likely the first example of high-$z$ gas clouds that have
preserved the chemical yields of Pop III stars exploding as low-energy SNe
[see 134]. However, we are still missing distant absorbers imprinted by the
chemical products of more energetic hypernovae and PISNe, whose direct
descendants have been already identified among metal-poor stars [121, 141,
142, 177]. Thanks to our Pop III diagnostic and the ANDES spectrograph we will
be able to finally discover these absorbers in the distant Universe.

Figure 4: Simulated ANDES spectrum of a metal absorption system at $z=7.0$,
with $\mathrm{N}(\mathrm{O~{}I})=10^{13}$ cm-2 along the line of sight to a
background $m_{\rm AB}=20$ quasar observed for 2 hours (noise level based on
the ANDES v.1.1 exposure time calculator). The panels display zoomed-in
regions around the OI $\lambda$1302, SiII $\lambda$1260 and CII $\lambda$1334
absorption features in three different scenarios enriched by the ejecta from:
the core collapse supernova (CCSN) of a $Z=1/3Z_{\odot}$, $m_{*}=20\
M_{\odot}$ star [black line; 145, yields], the Pop III CCSN of a $m_{*}=10\
M_{\odot}$ star [red line; 73, yields] and the Pop III PISN from a $m_{*}=204\
M_{\odot}$ star [blue line; 72, yields]. The three scenarios can be clearly
distinguished from the ANDES spectrum.  Figure 5: Relative abundances
corrected for dust depletion, $[X/\mbox{Fe}]_{\rm nucl}$, observed in
individual gas components/clouds (one per panel) associated with a $z=6.3$
GRB, observed with VLT-XSHOOTER [adapted from 132]. In grey, nucleosynthetic
yields for type II SNe [92] are reported as a function of different progenitor
masses $m_{*}=13,20,30\,M_{\odot}$ (respectively solid, dashed, and dotted
lines) for a chosen metallicity $\log(Z/Z_{\odot})=-1.3$. Similar studies will
be extended to numerous targets thanks to an instrument like ELT-ANDES.

###  3.3 Relative chemical abundances in gas at $z\gtrsim 6$

At higher redshift, the metallicity of DLAs decreases [e.g., 33] and the
probability of finding gas enriched by a single generation of stars increases.
Recent surveys are finding an increasing number of quasars around the
reionisation epoch, which can be used to study the gas in absorption [48, 41].

Because of the increasing cosmic mean density and neutral gas fraction, above
$z\sim 6$ it is no longer possible to measure the neutral hydrogen column
density of individual absorption features and therefore measure the absorbers’
metallicity. Nevertheless, the study of the relative abundances of elements
heavier than helium can be used in a powerful way to characterise the metal
pattern and thus the chemical enrichment of the absorbing gas. This is
illustrated in Figure 4, where we show simulated ANDES spectra of a $z=7.0$
absorber characterized by a neutral oxygen column density
$\mathrm{N}(\mathrm{O~{}I})=10^{13}$ cm-2. Here, we consider three different
single-SN enrichment scenarios for the absorber – two featuring core-collapse
SNe from either a metal-enriched or metal-free (Pop III), moderate-mass
($10-20$ $M_{\odot}$) progenitor star, and one featuring a PISN from a very
massive ($\approx 200\ M_{\odot}$) Pop III progenitor. By measuring the
strengths of the O$\,\rm\scriptstyle I\ $$\lambda$1302, Si$\,\rm\scriptstyle
II\ $$\lambda$1260 and C$\,\rm\scriptstyle II\ $$\lambda$1334 Å absorption
features in the intervening gas cloud, ANDES can readily distinguish the three
cases and probe the existence of very massive Pop III stars in the early
Universe.

The relative abundances in gas are also sensitive tracers of the depletion of
metals into dust, as well as to the nucleosynthetic signature imprinted in the
gas by the previous stellar populations. These effects can be well
disentangled by studying the relative abundances of different metals having
different refractory and nucleosynthetic properties [e.g 32]. This is
important because significant amounts of dust have been observed out to the
reionisation epoch [167, 132], so that dust depletion needs to be excluded or
accounted for to be able to unveil the often subtle effects of nucleosynthesis
of specific stellar populations. Figure 5 shows the relative abundances in
individual gas components/clouds within and around the host galaxy of a
$z=6.3$ GRB [132]. After correcting for dust depletion [e.g. 132, 34], the gas
relative abundances can be directly compared to stellar yields. This
comparison is displayed in Fig. 5, where some of the gas clouds/components
show high levels of $\alpha$-element enhancements, and a high [Al/O], which is
not well reproduced by the models.444Note that the H i column density of
individual components and, as a consequence, their metallicity, cannot be
determined observationally. Consequently, we have adopted the model yields by
Kobayashi et al [92] for a metallicity $\log(Z/Z_{\odot})=-1.3$ (corresponding
to an absolute metallicity $Z=0.001$) close to the overall metallicity of the
host galaxy ([M/H]${}_{\rm tot}=-1.72\pm 0.13$; Saccardi et al 132).

##  4 Extragalactic Transients

Extragalactic transients such as GRBs, various types of supernovae, Novae,
counterparts of gravitational wave events, and tidal-disruption events
unexpectedly light up superposed on the light of their host galaxy, and offer
us for a limited amount of time new insights into the physical nature of these
stellar- or galactic phenomena, and the galaxies that host them. This provides
a complementary study of galaxies since the detection of transients does not
depend on the host galaxy luminosity. The current limitation to these studies
is the faintness of most targets, which ANDES will overcome with the
unprecedented sensitivity of the ELT and with its near-IR optimised
capabilities.

###  4.1 Gamma-ray bursts

GRBs are the most energetic events after the Big Bang. GRBs lasting longer
than about 2 seconds are predominantly associated with the explosions of fast-
rotating massive stars [77, 175]. Their afterglows can be used as bright
cosmic beacons even at very high redshifts to unveil the chemical and
kinematical properties of the galaxies that host them based on rest-frame UV
absorption-line spectroscopy [e.g. 159, 122, 31, 20]. Absorption line spectra
typically trace the warm and cold, predominantly neutral ISM in the host
galaxy at distances greater than $\sim$100 pc from the explosion sites [160].

While GRB afterglows have been studied since 1997, numerous questions remain
unanswered, and even to this date in 2023, the diversity of events remains a
puzzle, including exceptionally long-duration or bright events, and compact
object mergers [e.g. 87, 99, 171]. ANDES will provide a leap forward with a
rapid acquisition of transient sources in the near-IR, combined with a wide
spectral coverage at a high resolution.

Searches for the afterglows conducted in the visible spectral range miss about
half of them [e.g. 53, 123, 54]. We know that X-ray afterglows are often
associated with bright, more metal-rich and dustier hosts than the optically
detected afterglows [119]. There hence is a large potential for spectroscopy
of these dusty sight-lines with the near-IR coverage of ANDES.

Currently, only a few GRBs at $z\gtrsim 6$ have very detailed chemical
information [71, 132]. The current record for a GRB with a secured afterglow
spectrum is $z=8.2$ [149, 135], with another explosion at photometric redshift
$z=9.4$ [30]. Long duration GRBs explode in star-forming galaxies and hence
the highest redshift explosions will identify the exact location of the
earliest generation of stars. Studying the absorption lines imprinted in the
spectra of GRB afterglows with ANDES will allow a crucial test on the chemical
yields in the early universe as described in Section. 3. Combining the ISM
metallicity detected in absorption with emission-line based galactic
metallicities would allow us to accurately calibrate the latter diagnostics,
which remains a challenge to date even at intermediate redshifts [136].
Furthermore, the estimate of the IGM neutral hydrogen fraction, $x_{\rm HI}$,
from the measured Lyman-$\alpha$ damping wing in high-redshift afterglows is
less complex compared to quasar spectra (see Sec. 2) due to the simple power-
law spectral distribution of afterglows, even in the presence of the
contribution of the host galaxy DLA [71]. Fig. 6 illustrates a simulated
$J$-band spectrum of a $z=8.2$ GRB with a 1% solar metallicity and a strong
damping wing from an intrinsic DLA in a fully neutral IGM. The simulated
spectrum shows the signal that ANDES will obtain in a hour integration on a
source with a magnitude $J_{\mathrm{AB}}$=20 in the nominal $R$=100,000
resolution, and an $R$= 50,000 mode.

Figure 6: Simulated ANDES $J$-band spectrum of a simulated GRB afterglow with
a magnitude of $J$=20 at $z=8.2$ with strong damping wing from a fully neutral
IGM combined with a strong intrinsic hydrogen Ly$\alpha$ damping wing and with
metal absorption lines corresponding to a 1% solar metallicity.

With VLT-UVES, astronomers made a breakthrough in discovering variable fine-
structure lines powered by the photo-excitation and ionizing effect of the GRB
afterglows on the surrounding gas, that allowed the determination of distances
of the absorbing clouds out to a kpc from the GRB [e.g. 160, 35, 31, 161].
Such studies are challenging because the afterglow fades quickly, and it is
impossible to detect the lines at phases later than one day after the burst.
With a higher sensitivity, ANDES will overcome this main limitation and detect
the line-variability as the GRB afterglows fade in time. This provides a
unique opportunity to probe the sizes and internal ISM structure of high
redshift galaxies, including that in very faint galaxies.

###  4.2 Superluminous Supernovae

Superluminous Supernovae (SLSNe) are a class of extremely energetic SNe [125,
57, 58], which are at least 10 and often 100 times more luminous than normal
SNe [e.g. 115, 80, 33]. Because of their brightness, they can be observed out
to high redshifts [currently $z\sim 4$, 25]. They are particularly intriguing
because their explosion processes and progenitor stars are highly debated, and
may be associated with very massive stars, with potential links or analogies
to pair-instability SNe and Pop III stars [59, 73]. The host galaxies of SLSNe
have low metallicities and extreme properties [114, 162, 98, 103, 118, 138].
Spectral observations of SLSNe with 8-10m telescopes are already photon-
starved and limited to mostly low-resolution studies, with a few exceptions
that unveil host-galaxy ISM environments that are very different from any
other galaxies we know of [162, 178]. In addition, SLSNe have a potential use
for cosmology out to much higher $z$ than currently done with SNe Type Ia [79,
139]. SLSNe spectra are diverse and fundamental to assess the properties of
the explosions [126], as well as their host galaxies in absorption [162, 178].
Current time-domain survey facilities such as the Zwicky Transient Facility
[ZTF, 64] discover about 30 SLSNe per year [23, 24].

In the future, ANDES will be ideal to follow up selected SLSNe out to high
$z$, and will bring new insights into chemical properties of the extreme host
galaxies. In addition, the observations will also bring important information
on the physics and diversity of these energetic explosions, the stellar
evolution of most massive stars and an intriguing perspective for SLSN
high-$z$ cosmology.

Figure 7: A limited wavelength range, covering the region nearby
Ca$\,\rm\scriptstyle II\ $H&K lines, of the spectrum of V906 Car obtained on
March 24, 2018, with VLT-UVES, rescaled at the resolution of ANDES, $R\sim
100,000$. A bunch of THEA lines, including Zr ii and Sr ii, are clearly
identified thanks to the resolution available with ANDES, and reported in the
plot.

###  4.3 Novae

Classical novae (CNe) are non-disrupting thermonuclear explosions happening on
the surface of a white dwarf that is accreting matter from a companion late-
type main sequence star [18, 36]. These cataclysmic events lead to the
ejection of the accreted shell into the interstellar medium, giving rise to
multi-wavelength emission visible for months, if not years, after the
outburst. Very high-energy emission in $\gamma$-rays has recently been
discovered [2] from the presence of shocks originating from the multi-
component nature of classical novae ejecta [113]. Evidence for internal shock
interaction, simultaneously to $\gamma$-ray and optical flares, within nova
outflows has been observed in high-resolution spectra due to the appearance of
new absorption systems and the evolution of P-Cygni absorption lines of the
main line transitions detected in CN spectra [7], including transient heavy
element absorption [172]. Untangling multiple components expanding at
different velocities within the nova ejecta [6] will help understand the still
unknown origin of the shock interactions giving rise to the high-energy
emission.

The resolution provided by ANDES will also allow us to identify and measure
the abundance of narrow transient heavy element absorption components (namely
THEA, Williams et al 172), which are generally low-ionisation transition of
neutral and ionised Fe-peak elements, including heavier elements such as Sr,
Y, and Ba (see Fig. 7. These narrow lines have been detected during the bright
optically thick phases of several CNe, and they exhibit only a single, low
velocity, absorption component, suggesting that they are located in the slow
toroidal ejecta component, but whose origin is not clearly identified yet.
Among these lines, we have also identified in few cases the presence of the
resonance line of the neutral lithium (Li i at 670.8 nm), which has been
theoretically predicted to be synthesised in novae already in the ’70s [4,
144], but was only discovered in CNe recently, thanks to the use of very high-
resolution spectrographs [82]. ANDES will enable us to measure the abundance
of THEA lines, compare their inferred abundance pattern with the spectra of
late-type main sequence stars that are similar to the donor secondary star,
and shed light on their origin. At the same time, detection of lithium will be
crucial to determine the lithium yield from a large sample of novae, possibly
simultaneously to observations in the near-UV to measure the abundance of its
parent element (7Be ii at 313.0 nm), and finally to understand and
characterise the role of novae as one of the main factories of lithium in the
Galaxy [112] and in nearby systems such as the Magellanic Clouds [83].

###  4.4 Specific requirements for the transient science cases

The majority of extra-galactic transient science investigations focus on
spectroscopic analysis of individual objects, where stringent atmospheric
conditions are not necessary, and can be conducted with seeing-limited
observations. The ability of ANDES to simultaneously cover a wide wavelength
range from 0.4 to 1.8 microns in a single exposure offers a significant
advantage. Observations of extragalactic transients need the availability of a
target of opportunity (ToO) mode, while the rapidly fading GRB afterglows
require a rapid response mode (RRM) similar to that available on the VLT.
Particularly, catching the fading afterglow minutes from the GRB trigger will
benefit from an enhanced sensitivity enabled by a lower resolution ($R\approx
50,000$) mode (see Section 7).

##  5 Additional science cases

The science cases described in the previous sections were chosen by the ANDES
science team as the most compelling to be carried out with this instrument in
the field of galaxy formation and evolution and the intergalactic medium.
However, they are far from exhausting the extragalactic science that will be
enabled by ANDES.

All the studies devoted to the investigation of the physical and chemical
properties of the IGM, CGM and ISM at $z\gtrsim 2.5-3.0$ will be improved in
terms of statistics (number of targets), sensitivity as a function of exposure
time and details of the velocity profile of absorption lines thanks to the
characteristics of ELT-ANDES (but see also Sect. 6). Furthermore, ANDES could
be used to observe pairs or small groups of quasars at small angular
separations ($\lesssim 1$ arcmin) on the sky to determine the small-scale
variation of chemical properties in the CGM (see also Sect. 8 for the synergy
with other ELT instruments).

In the following, we briefly describe three example science cases further
describing the capabilities of ANDES.

###  5.1 He$\,\rm\scriptstyle II\ $reionisation

In the cosmic reionisation theory, young stars play a key role in the
reionisation of H i at $z\gtrsim 6$ [e.g. 47], but they are typically unable
to further doubly ionise helium. Instead, quasars are thought to be the most
likely ionizing sources responsible for the He$\,\rm\scriptstyle II\
$reionisation, which started at $z\gtrsim 5$ and completed at $z\gtrsim 3$
[e.g. 109]. The C$\,\rm\scriptstyle IV\ $absorption doublet at rest frame
$\lambda\lambda$1548.2,1550.8$\rm~{}\AA$ in the spectra of bright background
quasars at $z\gtrsim 4$ is the best tracer of the He$\,\rm\scriptstyle II\
$reionisation [e.g. 182]. This is because the C$\,\rm\scriptstyle IV\ $doublet
is strong and easy to identify, located at significantly longer wavelengths
than the hydrogen Lyman-$\alpha$ emission line in the quasar spectrum, and the
C3+ ion has a comparable ionization potential as He+. Using high resolution
optical spectra, after firmly identifying the C$\,\rm\scriptstyle IV\
$absorbers, it is then easier to identify other associated metal absorbers
such as Si$\,\rm\scriptstyle IV\ $[e.g. 40], as well as the Lyman-$\alpha$
lines embedded in the forest [e.g. 140]. The identification of these multi-ion
systems will help us to study the physical and chemical properties of the IGM
in the He$\,\rm\scriptstyle II\ $reionisation epoch (e.g., Simcoe et al 140,
Songaila 143), when the AGN become more and more important in the cosmic
reionisation (e.g., Faucher-Giguère et al 49), and some large-scale
gravitationally bound systems gradually become virialized (e.g., Wang et al
166, Tozzi et al 152). High-resolution optical spectroscopy observations of
$z\gtrsim 4$ quasars with $\sim 8\rm~{}m$ telescopes typically allow the
detection of C$\,\rm\scriptstyle IV\ $absorbers in $i<19\rm~{}mag$ quasar
spectra to an equivalent width of EW${}_{\rm r}\gtrsim 0.01$ Å (corresponding
to $\log(N{\rm(C{\small~{}IV})/cm^{2}})\gtrsim 12.5$; e.g., Cooksey et al 28,
D’Odorico et al 40). This leads to a small sample size due to either a small
number of bright enough quasars (seriously affecting the statistics at the
high $N\rm(C{\small~{}IV})$ end) or a small number of C$\,\rm\scriptstyle IV\
$absorbers above the currently reached detection limits. With the high-
resolution mode of ELT-ANDES ($R\sim 100,000$), it will be much more efficient
to reach a lower C$\,\rm\scriptstyle IV\ $column density detection limit of
$\log N{\rm(C{\small~{}IV})/cm^{2}}\lesssim 12$ (assuming $i\approx
20\rm~{}mag$, $\rm exposure\approx 2\rm~{}hours$, $\rm S/N\sim 27$). This is
critical to constrain the C$\,\rm\scriptstyle IV\ $column density distribution
function and its redshift evolution (e.g., Ellison et al 43, D’Odorico et al
38). Furthermore, if a lower resolution, higher throughput mode of ANDES at
$R\sim 50,000$ will be implemented, an increase in sensitivity of about a
factor of two is expected, which implies that the same threshold in
C$\,\rm\scriptstyle IV\ $column density could be reached in the same exposure
time for targets $\sim 1.0-1.5$ magnitudes fainter.

###  5.2 Probing the circumgalactic medium of local galaxies

The Ca$\,\rm\scriptstyle II\ $H&K lines near 3950 Å are the only lines in the
optical that can be used to study the cold/warm CGM in absorption at $z\approx
0$. Previous surveys of Ca$\,\rm\scriptstyle II\ $absorption in the CGM of the
Milky Way and other low-redshift galaxies using VLT-UVES and Keck-HIRES [14,
128, 15] demonstrate that these lines are particularly useful to study the
small-scale structure at pc and sub-pc levels in the neutral CGM down to
neutral gas column densities of log $N$(H i$)=18$.

ANDES and its planned spectral capabilities in the U-band will initiate a new
era in using the Ca$\,\rm\scriptstyle II\ $H&K lines to characterize the
internal density structure in $z\approx 0$ CGM clouds. In principle, every
extragalactic spectrum recorded with ANDES in the U-band can be used to
investigate the foreground CGM of the Milky Way, tidal gas structures in the
Local Group (e.g., the Magellanic Stream), and other circumgalactic gas
features of foreground galaxies. The systematic comparison of
Ca$\,\rm\scriptstyle II\ $H&K absorption patterns as a function of the lines
of sight angular separations along thousands of them will provide crucial
information on the small-scale distribution and coherence length of neutral
gas structures in the (multi-phase) CGM.

Spectra of extragalactic sightlines can also be systematically combined with
ANDES U-band spectra of (in projection) adjacent, distant halos stars [8] or
stars in other Local Group member galaxies to constrain the distances of the
Milky Way’s high-velocity clouds and the Magellanic Stream using the distance-
bracketing method [e.g. 163, 164, 150]. This will be an essential step to
reconstruct the 3D distribution and total mass of the Milky Way’s CGM, to
determine the Milky Way’s gas-accretions rate, and to pinpoint the exact
location and extent of the Magellanic Stream in the context of the on-going
assembly of the Local Group.

With its capabilities to obtain high-resolution optical spectra of
extragalactic background sources at extremely high S/N ($>2000$), ANDES also
will potentially open a completely new window to study warm-hot gas in the
intergalactic medium in the halos of foreground galaxies at low redshift by
using the forbidden intersystem lines of [Fe x] and [Fe xiv] near 6375 and
5303 Å. As originally proposed in York and Cowie [180], and later re-evaluated
by other groups [e.g. 78, 120, 129], these extremely weak lines have a great
potential to be used as tracers of million-degree halo gas at the viral
temperatures of (massive) galaxies and hot gas in the shock-heated component
of intergalactic medium (the WHIM). This requires, however, optical spectra of
background AGN with a S/N of several thousand because of the extremely small
transition probabilities of [Fe x] and [Fe xiv]. The combined use of
individual ANDES high-sensitivity spectra and clever stacking strategies will
enable us to search for warm-hot gas using [Fe x] and [Fe xiv] in different
cosmological environments and to provide a census of the “missing baryons” in
the WHIM in the local Universe [51].

###  5.3 Low-mass Black Holes

Supermassive black holes (BH; M${}_{\rm BH}\sim 10^{5}-10^{10}$ M) are key
actors in the formation and evolution of galaxies. The feedback due to
radiation and winds from the accretion disk surrounding the BH, can shape the
host galaxies and explain the present-day empirical scaling relations between
their properties and the BH mass [e.g. 46, 95, 90], and possibly also the
dearth of massive galaxies [e.g. 13]. However, this paradigm is ultimately
mostly based on several tens direct BH mass measurements in massive early type
galaxies with $M_{\rm BH}>10^{7}$ $M_{\odot}$ and little is known for the less
massive galaxies which are predicted to host BH with $M_{\rm BH}<10^{7}$
$M_{\odot}$ [see e.g. 95, 67].

It is currently believed that low mass galaxies undergo a different growth
than massive ones and, as a result, the distribution of their BH masses should
retain an imprint of the original seed mass distribution [e.g. 158, 127]: BH
growth models predict significantly different $M_{\rm BH}-\sigma$ relations at
$M_{\rm BH}\sim 10^{5}-10^{6}$ $M_{\odot}$, for different initial mass
functions of the seed BH, i.e. for different seed formation mechanisms. JWST
has started to probe this BH mass regime at high redshift [93, 70, 105, 104],
but in type 1 AGNs and using the so-called virial relations and single epoch
observations, whose validity at high redshift (and specifically for the class
of highly accreting BH identified by JWST) is uncertain.

To properly test the BH seeding scenarios, direct measurements of BH masses in
the range $\sim 10^{5}-10^{6}$ $M_{\odot}$ are needed. This is feasible in the
local universe, provided that one has high enough spectral resolution and high
enough angular resolution. A standard approach based on the measurement of gas
and/or stellar kinematics would require to spatially resolve the BH sphere of
influence which, for a $\sim 10^{5}$ $M_{\odot}$ BH, in a galaxy with central
stellar velocity dispersion of about 50 km s-1 located at distance $D\sim 20$
Mpc (e.g., in the Virgo cluster), has a projected diameter of 4 mas. The
diffraction limited spatial resolution of the ELT at 1.2 $\mu$m, 2.2 $\mu$m
(e.g. at the location of Paschen-$\beta$, Br$\gamma$, H2) is 6 and 12 mas
respectively, meaning that even the ELT cannot reach the required spatial
resolution.

Spectroastrometry of gas emission lines is a method complementing the
classical techniques and which, by exploiting high spectral resolution, allows
to measure gas rotational velocities on scales as small as 1/10 of the spatial
resolution. This method consists in tracing the spatial centroid of the
emission line in many independent spectral channels: if the gas is circularly
rotating in a disk, then the centroid will present a unique variation with
wavelength which can be modelled to provide the mass of the central BH. The
method has been tested and successfully used to measure BH masses [61, 62,
63]. The same method has been successfully adopted in interferometric
observations of the quasars 3C273 with VLTI-Gravity to spatially resolve, for
the first time, the Broad Line Region and revel its rotation [65].

By reaching even only 1/5 of the spatial resolution of the ELT, with
spectroastrometry it will be possible to spatially resolve scales down to 1
and 2.5 mas at $\lambda\sim 1.2$ $\mu$m and 2.2 $\mu$m, respectively.
Spectroastrometry with the ELT will therefore allow us to spatially resolve
the sphere of influence of $10^{5}$ $M_{\odot}$ BHs at a distance of 20 Mpc.
To adequately sample the rotation curve of a $10^{5}$ $M_{\odot}$ BH at $D\sim
20$ Mpc, at least 10 spectral channels should be sampling the nuclear rotation
curve ($30-60$ km s-1), therefore the spectral resolution must be
$(60-30)/10/2=3$ km s-1, corresponding to R $\sim 100,000$. To adequately map
the centroid position of the central unresolved source an IFU sampling the
diffraction limited ELT PSF is needed. Complete near-IR spectral coverage will
allow the use of several gas emission lines, Pa-$\beta$ in J, [FeII] in H, and
H2 and Br-$\gamma$ in K. In summary, an IFU mode for ANDES working at the
diffraction limit of the ELT and a spectral resolution of $\sim 100,000$ would
be able to detect BHs with masses down to $10^{5}$ $M_{\odot}$ up to a
distance of $\sim 20$ Mpc (i.e. Virgo).

##  6 The benefits of the ANDES extension to the U-band

In Section 5.2, we highlight how the extension of ANDES to the U-band
($0.35-0.4$ $\mu$m) would be of fundamental importance for the study of the
CGM of the Milky Way and of the gas distribution in the Local Group. More
generally, we would like to stress that almost the entire field of diffuse gas
studies (IGM, CGM and ISM) over the Universe’s history is based on absorption
lines from electronic transitions, the vast majority of which occurring in the
far UV. An extension to the U-band then corresponds directly to an extension
in available redshifts. For instance, the study of gas metallicity requires
access to H i lines, and studying the transition to cold gas and interplay
with stars (as well as AGN activity) requires covering H2 lines. For systems
at the raise and peak of star-formation history, these lines will be
redshifted in the U-band.

##  7 The benefits of a higher sensitivity, lower resolving power mode

The majority of the science cases proposed in this work rely on the detection
and measurement of spectral features which are generally resolved at a
resolving power of $R\sim 50,000$. The comparison of quasar metal absorption
lines observed at $R\gtrsim 100,000$ and $R\sim 50,000$ shows how the velocity
profile of complex absorption systems does not change substantially with the
increase in resolution, although the different velocity components (and in
particular the narrow ones) become more visible [see e.g. 16].

One of the aims of ELT-ANDES is to push the limits of the study of chemical
enrichment to the early universe: this requires fine sensitivity. At $z\sim 6$
most quasars have magnitudes fainter than $J_{\rm AB}\sim 19$ [41] and the
brightest GRB afterglows at $z\sim 6$ have magnitudes around $m_{i}\sim 20$
mag that fade within hours [71, 148]. Targets at higher redshift will be
fainter. An intermediate resolution mode ($R\sim 50,000$) on ANDES will enable
a $\sim 60$% higher sensitivity than with the full-resolution mode, in the J
band. This will be crucial to be able to reach the sensitivity levels needed
to study the faint high-$z$ sources.

Simulations of GRB afterglow spectra show that the expected fraction of the
total population of GRBs at $z>6$ is $\sim 6$% [60]. Their magnitude 0.5 days
after the GRB trigger in the observer frame is in the range $17-27$ mag (AB)
in the $J$-band, with a fraction of 20%(35%) of afterglows being brighter than
20(21) mag. In case of an RRM trigger just 1 hour after the explosion,
60%(85%) are brighter than 20(21) mag. Requiring a S/N $>10$ per spectral
pixel to analyse absorption lines at the nominal resolution of $R$=100,000, a
single 50 minutes integration on a mag=20 source is adequate, while a 21 mag
source requires 4 hours on target555 Based on Version 1.1 of the ANDES
Exposure Time Calculator.

In summary, a lower spectral resolution mode ($R\sim 50,000$), which
additionally benefits from an improved instrument throughput, is highly
desirable because it would increase the sensitivity of ANDES by a factor of
$\sim$2 compared to the baseline instrument configuration.

##  8 Synergy with present and future surveys and facilities

###  8.1 Selection of ANDES targets

Several upcoming telescopes and surveys will be important for optimising the
selection of targets for ANDES. This includes Euclid, for which the Euclid
Wide Survey [45] is expected to extend the available sample of bright
($m_{\mathrm{AB}}<22$) $z>7$ quasars [44, 137], and the Legacy Survey of Space
and Time (LSST) at the Vera C. Rubin Observatory [81], which will allow the
detection of SLSNe up to $z\approx 5$ [157]. The Dark Energy Spectroscopic
Instrument (DESI) has recently added several hundred objects to the known set
of $z\approx 5$–7 quasars at $m_{\mathrm{AB}}\lesssim 21$ in the southern
hemisphere, and is projected to double their sample in coming years [179]. In
the same redshift range, the future 4MOST survey is moreover expected to find
$\sim 4\times 10^{4}$ quasars at $z>5$ and $m_{\mathrm{AB}}\lesssim 22.5$
[110]. On a longer horizon, the planned ESA THESEUS [3] or NASA Gamow Explorer
[170] missions may allow the detection of $z\gtrsim 6$ GRBs with afterglows
that for a few hours will remain sufficiently bright for follow-up
observations with ANDES [60].

###  8.2 ANDES complemented by other instruments and future facilities

The aspiration of ANDES to detect very metal-poor gas clouds and the chemical
signatures of Pop III stars in absorption offers strong synergies with other
optical and infrared spectrographs that will operate on ELT and that are
currently operating on JWST. Follow-up observations of quasar fields hosting
relevant absorbers will be possible at, e.g., ELT with the MOSAIC
spectrograph. This instrument will be vital in identifying both galaxies
associated with the absorbers, including the host galaxies, and characterizing
the larger-scale galaxy environment near this gas by reaching low stellar-mass
systems. The prospects of ANDES for studying low-metallicity gas at high
spectral resolution along the line of sight to quasars fainter than what can
currently be reached will thus be well-matched to upcoming ELT capabilities to
probe the faint end of the galaxy population that clusters with these gas
absorption systems.

While ANDES will be able to characterise the distribution and occurrence of
absorbing gas clouds that bear the chemical imprints of the earliest
generations of stars, JWST is already searching for Pop III-powered galaxies
that lack strong metal emission lines but exhibit very strong hydrogen and
helium emission lines [see early results by 156]. In the future, it may be
possible to combine the two approaches and use JWST to search the fields
around absorbers that exhibit signatures of Pop III enrichment, thereby
potentially identifying the galaxies from which the supernova enrichment
originated and constraining the outflow of metals from sites of recent PopIII
star formation.

In the study of the high-redshift CGM with ELT, the combined use of the
HARMONI/MOSAIC instruments for the fainter, more numerous targets and of ANDES
for the brighter quasars in the same field will moreover allow us to
reconstruct the 3D distribution and physical properties of the gas. At lower
redshifts, the tomographic mapping of CGM in Ca ii from ANDES with H i 21cm
data from SKA will furthermore boost our understanding of the large-scale and
small-scale neutral gas distribution around galaxies and its role for galaxy
evolution.

In the study of cosmic reionisation, observations of the redshifted 21 cm
signal from neutral hydrogen with SKA will allow new ways to connect small-
scale (ANDES) and large-scale (SKA) constraints on the redshift evolution of
the reionisation process. For rare high-redshift quasars or GRB afterglows
that appear sufficiently bright at both near-IR and radio wavelengths,
combined detections of metal (ANDES) and 21 cm neutral-hydrogen (SKA)
absorption lines along the same sightlines may furthermore be possible [17].

Acknowledgments A.D.C. acknowledges support by the Swiss National Science
Foundation under grant 185692. This project has received funding from the
European Research Council (ERC) under the European Union’s Horizon 2020
research and innovation programme (grant agreement No 757535) and by
Fondazione Cariplo (grant No 2018-2329). EZ acknowledge funding from the
Swedish National Space Board and project grant 2022-03804 from the Swedish
Research Council (Vetenskapsrådet), and has benefited from a sabbatical at the
Swedish Collegium for Advanced Study.

## Declarations

  * •

Authors’ contributions
VD: Coordination, general writing and finalization of the paper; JSB:
Coordination and writing of Sect. 2; ADC: Coordination and writing of Sect. 3;
LC: Coordination and writing of Sect. 4; EZ: Writing of Sect. 8; SS: Writing
of Subsect. 3.1; LI: Writing of Subsect. 4.3; JL: Writing of Subsect. 5.1; PR:
Writing of Subsect. 5.2; RM, AML: Writing of Subsect. 5.3; AK, AS, IV: Help
with simulations/forecasts, figures; PDM, RM, AM, LO, AZ: ANDES project office
support; All authors provided comments to the draft.

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