# Milky Way and Nearby Galaxies Science with the Single Aperture Large
Telescope for Universe Studies (SALTUS) Space Observatory

Rebecca C. Levy  Department of Astronomy and Steward Observatory, University
of Arizona, Tucson, AZ, 85721, USA  Alexander Tielens  Leiden Observatory,
P.O. Box 9513, Leiden, The Netherlands  Astronomy Department, University of
Maryland, College Park, MD 20742, USA  Justin Spilker  Department of Physics
and Astronomy and George P. and Cynthia Woods Mitchell Institute for
Fundamental Physics and Astronomy, Texas A&M University, 4242 TAMU, College
Station, TX 77843-4242, USA  Daniel P. Marrone  Department of Astronomy and
Steward Observatory, University of Arizona, Tucson, AZ, 85721, USA  Desika
Narayanan  Department of Astronomy, University of Florida, 211 Bryant Space
Sciences Center, Gainesville, FL 32611 USA  Cosmic Dawn Center at the Niels
Bohr Institute, University of Copenhagen and DTU-Space, Technical University
of Denmark  Christopher K. Walker  Department of Astronomy and Steward
Observatory, University of Arizona, Tucson, AZ, 85721, USA

###### Abstract

This paper presents an overview of the Milky Way and nearby galaxies science
case for the Single Aperture Large Telescope for Universe Studies (SALTUS)
far-infrared NASA probe-class mission concept. SALTUS offers enormous gains in
spatial resolution and spectral sensitivity over previous far-IR missions,
thanks to its cold ($<$40 K) 14-m primary mirror. Key Milky Way and nearby
galaxies science goals for SALTUS focus on understanding the role of star
formation in feedback in the Local Universe. In addition to this science case,
SALTUS would open a new window to to of Galactic and extragalactic communities
in the 2030s, enable fundamentally new questions to be answered, and be a far-
IR analog to the near- and mid-IR capabilities of JWST. This paper summarizes
the Milky Way and nearby galaxies science case and plans for notional
observing programs in both guaranteed and guest (open) time.

###### keywords:

mission concept, Galactic science, Extragalactic science, THz spectroscopy,
far-infrared, Heterodyne resolution

†National Science Foundation Astronomy & Astrophysics Postdoctoral Fellow

*Rebecca C. Levy<EMAIL_ADDRESS>
##  1 Introduction

The Single Aperture Large Telescope for Universe Studies (SALTUS) observatory
is a far-infrared (FIR) mission concept proposed to NASA’s recent APEX call
for proposals 1. Its unique 14-m primary mirror and sensitive FIR ($\approx
30-700$ $\mu$m) instrumentation would provide orders-of-magnitude improvements
over past and other proposed FIR space missions. The very large primary mirror
size results unprecedented $\sim$1′′ spatial resolution in the FIR, making
SALTUS a well-matched bridge in wavelength between the James Webb Space
Telescope (JWST) and the Atacama Large Millimeter/submillimeter Array (ALMA).
The overall telescope, receivers, and spacecraft architectures are described
in detail in a series of papers elsewhere in this issue2, 3, 4. Briefly, the
two instruments planned for SALTUS are SAFARI-Lite and HiRX. SAFARI-Lite is a
direct detection spectrometer providing medium resolution ($R\sim 300$)
spectroscopy over the entire $34-230$ $\mu$m wavelength range simultaneously5.
HiRX is a multi-pixel, multi-band heterodyne receiver ($R\sim 10^{5}-10^{7}$),
enabling sub-km s-1 spectroscopy of key spectral lines for interstellar medium
(ISM) science6.

This paper provides an overview of the Milky Way and nearby galaxies science
uniquely enabled by SALTUS. Accompanying papers in this issue describe the
capabilities of SALTUS for high-redshift galaxies7, star and planet
formation8, and solar system observations9. In the remainder of this section,
we provide background on the key principles and tracers of processes in the
ISM of galaxies and the Milky Way. Section 2 highlights the SALTUS performance
characteristics most relevant for Milky Way and nearby galaxies science. In
Section 3, we present several key measurements, enabled by SALTUS, that answer
key open questions in the fields of Milky Way and nearby galaxies science.

###  1.1 Stellar Feedback and the Ecology of Galaxies

Figure 1: The role of feedback and the lifecycle of galaxies. Stars form
inside dense cloud. The mechanical and radiative energy injected during their
evolution from the protostellar to the main sequence to the supernova phase
will heat interstellar gas, inject turbulence into the medium, disrupt
molecular clouds, and create the various phases of the ISM. During the
asymptotic giant branch phase and the supernova phase, newly synthesized
elements will be ejected, slowly enriching the ISM. The concerted action of
massive stars in rich OB associations will drive the formation of chimneys
that vent enriched gas into the lower halo and even the intergalactic medium.
Halo gas will condense in clouds that rain back onto the galactic plane. The
concerted action of these processes drives the evolution of galaxies. Figure
taken from 10.

Stellar feedback plays a central role in galactic ecology (Figure 1) and is a
key question posted in the 2020 Astronomy Decadal Survey (e.g., “How do gas,
metals, and dust flow into, through, and out of galaxies?” and “How do star-
forming structures arise from, and interact with, the diffuse interstellar
medium?”)10. During their formation phase, protostellar jets and winds stir up
their environment and greatly modify the structure of the clouds in which they
form and hence the star formation process. During their main sequence phase,
the interaction of massive stars with their environments regulates the
evolution of galaxies. Mechanical and radiative energy input by massive stars
stir up and heat the gas and control cloud and intercloud phases of the ISM.
The dominant mode of stellar feedback changes with the stellar/cluster mass
and as the star/cluster evolves11, 12, 13. Stellar feedback also governs the
star formation efficiency of molecular clouds14, 15, 16, 11. On the one hand,
stellar feedback can lead to a shredding of the nascent molecular cloud within
a few cloud free-fall times thereby halting star formation17, 18, 19. On the
other hand, massive stars may also provide positive feedback to star formation
as gravity can more easily overwhelm cloud-supporting forces in swept-up
compressed shells20, 21. The Spitzer/GLIMPSE galactic plane survey has
revealed hundreds of parsec scale bubbles associated with O and B stars,
attesting to the importance of stellar feedback in stirring up the ISM22. The
expansion of these bubbles may be driven by thermal pressure of the warm
ionized gas, radiation pressure by stellar photons, or the mechanical energy
input by stellar winds. The PHANGS survey has revealed larger (up to
$\approx$1 kpc) bubbles in nearby galaxies, likely driven by mechanical energy
from stellar winds and (clustered) supernovae23, 24. As massive star formation
dominates the energetics and feedback in star forming galaxies, properly
accounting for the star formation feedback is a critical ingredient of galaxy
evolution models and validating the subgrid feedback descriptions in these
hydrodynamic studies is of paramount importance.

###  1.2 Tracers of the ISM

Much of the interaction of massive stars with their environment occurs in
regions shrouded in dust and the far-infrared – where dust extinction is
minimal – provides many key diagnostic transitions that measure the physical
conditions in these regions and trace their evolution. Specifically, this
spectral range hosts atomic and ionic fine-structure transitions, covering a
wide range in critical density ($100-500$ K; $3\times 10^{2}-3\times 10^{4}$
cm-3)25.

The [NeIII] 36 $\mu$m, [SIII] 33.4 $\mu$m, [NII] 122 & 205 $\mu$m, [NIII] 57
$\mu$m, and [OIII] 52 & 88 $\mu$m lines trace ionized gas in HII regions
around hot young stars, and provide a measure of the gas density and the
hardness of the stellar radiation fields 26. As these transitions are not
temperature sensitive, galactic elemental abundance gradients can also be
accurately assessed with mid-IR HI recombination lines provided by JWST.

The physical conditions in photodissociation regions (PDRs) can be studied
through the [OI] 63 & 145 $\mu$m, [CII] 158 $\mu$m, [SiII] 35 $\mu$m fine-
structure transitions, as well as high-$J$ CO rotational transitions (e.g.,
$J=10-9$, 260 $\mu$m, through $J=19-18$, 137 $\mu$m)111The high sensitivity
will bring many higher order CO transitions into SALTUS purview, as well., in
addition to 13CO isotopes and the deuterated hydrogen (HD) molecule. All of
these transitions are known to be bright in PDRs27, 28, 29. Together, these
transitions span a wide range in critical densities ($\simeq 3\times
10^{3}-10^{6}$ cm-3) and excitation energies ($\simeq 100-500$ K) and thus
provide powerful tools to determine physical conditions in these neutral gas
regions. The strong far-UV irradiation of PDRs also leaves it imprint on the
chemical composition, leading to a strong stratification of the molecular
composition ranging from a surface layer characterized by small hydride
radicals222The first steps toward molecular complexity start with the
formation of simple hydride radicals. to the deeper zones dominated by heavier
species, including CO, HCN, etc.30. Because of their large moments of inertia,
light hydride radicals have their main transitions in the far-IR. Herschel has
detected strong emission from HD, OH, H2O, H2O+, H3O+, CH, CJ+, SH, HF, and
H2Cl+ in the Orion Bar 31.

The mechanical energy input by stars during the protostellar and main sequence
phase drive strong shock waves into the surrounding gas, sweeping them up in
dense shells. The far-infrared provides a convenient probe of the temperature
structure through the slew of CO pure rotational transitions accessible with
SAFARI-Lite and HiRX 32. The chemical processing of the gas by the shock
starts with the sputtering of ice mantles dominated by H2O and to a lesser
extent NH3. This is the first step in the build up of chemical complexity in
regions of star formation.

##  2 SALTUS Performance for Milky Way and Nearby Galaxies Science

The SALTUS telescope, instrumentation, and spacecraft design principles are
described in accompanying papers in this issue1, 2, 3, 4, 5, 6. Here we
briefly describe the performance metrics most relevant for the Milky Way and
nearby galaxies science community, and we point readers to those other papers
for detailed information. We envision that the SAFARI-Lite grating
spectrometer will be the workhorse instrument for this community.
Additionally, key spectral lines (e.g., water, [CII], etc) will be observable
with the HiRX heterodyne instrument at very high spectral resolution.

###  2.1 Wavelength Coverage

The SAFARI-Lite instrument provides $R\approx 300$ ($\Delta v\approx 1000$ km
s-1) spectroscopy over the entire $34-230$ $\mu$m wavelength range
simultaneously, covering the entire FIR at $z=0$ (Figure 2). To achieve this,
the wavelength range is divided into four co-aligned bands, each with six
spatial pixels, which are dispersed onto 180-detector MKID arrays. ISM heating
and cooling is governed by forbidden transitions of atoms and molecules which
occur in the FIR. The key line, and the brightest line in galaxies in the FIR,
is the 158$\mu$m transition of [CII]. This line will be covered by both
SAFARI-Lite and HiRX, enabling detection and characterization over a range of
environments and targeted high spectral resolution kinematic/energetic
studies. Other key lines covered by SAFARI-Lite include [SIII] 34$\mu$m,
[SiII] 35$\mu$m, [OI] 63,145$\mu$m, [OIII] 52,88$\mu$m, [NII] 122,205 $\mu$m,
and high-J CO transitions (Figure 2). Much more information about the SAFARI-
Lite instrument can be found in this issue5.

Figure 2: An example of the wavelength coverage of SALTUS compared to a
spectrum of the archetypal nearby starburst galaxy M 82 (black; from
Spitzer/IRS33, ISO/LWS34, and Herschel/SPIRE35). The SAFARI-Lite simultaneous
coverage is shown in blue and the HiRX bands are shown in purple. The coverage
of JWST/MIRI (light pink) and the ALMA bands (dark pink) are plotted for
reference. Key diagnostic lines are shown, tracing the ionized (red), neutral
(yellow), molecular (blue), ionized/neutral (orange), and
ionized/neutral/molecular (green) ISM phases; this list is not exhaustive.
With SALTUS, we will measure spectra of nearby galaxies at substantially
higher spatial resolution, spectral resolution, and sensitivity compared to
previous instruments, opening a fundamentally new window into the study of the
ISM in the local Universe.

The HiRX instrument provides sub-km s-1 spectral resolution in four tunable
bands chosen to cover key spectral features for a broad range of science cases
(Figure 2). Band 1 (521-659$\mu$m) covers two key CO rotational transitions
(J=5-4 and J=4-3) and a H2O ortho transition. Band 2 ($136-273$ $\mu$m) covers
key lines such as [NII] 122,205 $\mu$m, [OI] 145 $\mu$m, [CII] 158 $\mu$m,
several deuterated water (HDO) transitions, two prominent H2O lines, and
several high-J CO lines. Band 3 (112 $\mu$m) covers the ground state
rotational transition of deuterated molecular hydrogen (HD 1-0). Band 4
($56.2,63.2$ $\mu$m) covers HD 2-1 and [OI] 63 $\mu$m. Much more information
about the HiRX instrument can be found in this issue6.

###  2.2 Spatial Resolution

It is expected that SALTUS would be diffraction limited down to 30 $\mu$m,
achieving $0.5^{\prime\prime}-3.6^{\prime\prime}$ resolution over the
wavelength range covered by SAFARI-Lite and
$0.8^{\prime\prime}-9.7^{\prime\prime}$ resolution over the HiRX bands. As a
result, SALTUS will offer transformative gains in physical resolution for the
study of the Milky Way and nearby galaxies compared to the Herschel Space
Observatory ($\approx 16\times$ smaller beam area) or the Stratospheric
Observatory for Infrared Astronomy (SOFIA; $\approx 30\times$ smaller beam
area). For example, SALTUS will enable mapping of Milky Way sources, such as
Orion BN/KL at $\approx 200-1000$ AU spatial resolution, filling gaps between
ISM and star and planetary system formation science questions. For galaxies
within 20 Mpc (e.g., the PHANGS sample 36, 37), SALTUS enables mapping of
entire galaxies on $50-200$ pc scales, revealing the structure of the ISM and
sites of feedback.

###  2.3 Mapping Sensitivity

SALTUS’ large (14-m), passively cooled ($<40$ K) primary mirror offers
transformative increases in sensitivity over previous instruments1 (e.g.,
their Figure 4). The sensitivity of SAFARI-Lite is similar or better than that
of JWST/MIRI at the short wavelength end and ALMA at the long wavelength end.
SAFARI-Lite would be more than two orders of magnitude more sensitivity
compared to previous comparable instruments, such as the Photodetector Array
Camera (PACS) on Herschel38 and Field-Imaging Far-Infrared Line Spectrometer
(FIFI-LS) onboard SOFIA39.

###  2.4 Spectral Mapping Speed

Although SALTUS has a large primary mirror, it is able to map a $\approx$5
arcmin2 area without repointing by using a fine-steering motor. This map size
is similar to those of single JWST/NIRCam and MIRI images. Crucially, however,
these “maps” from SALTUS/SAFARI-Lite are three-dimensional, containing a
spectrum covering $34-230$ $\mu$m at each pixel, and therefore contain
significantly more information than single filter images from JWST. This
mapping area is sufficient to capture Milky Way star-forming regions and many
nearby galaxies in one or two pointings. Larger, more nearby targets, e.g., M
82, NGC 253, or Local Group galaxies, will need multiple pointings to observe
the entire disk, but a single pointing is sufficient to study particular areas
of interest (e.g., galactic centers, regions of high star formation, etc).

##  3 Key Milky Way and Nearby Galaxies Science Goals for SALTUS

As highlighted by the example science cases given in this section, SALTUS will
enable us to answer key questions about our Cosmic Ecosystem posed by the 2020
Astronomy Decadal Survey10, such as

  1. 1.

How do star-forming structures arise from, and interact with, the diffuse
interstellar medium?

    1. (a)

What sets the density, temperature, and magnetic structure of the diffuse ISM,
enabling the formation of molecular clouds?

    2. (b)

How do molecular clouds form from, and interact with, their environment?

    3. (c)

How does injection of energy, momentum, and metals from stars (“stellar
feedback”) drive the circulation of matter between phases of the ISM and CGM?

  2. 2.

What regulates the structures and motions within molecular clouds?

    1. (a)

What is the origin and prevalence of high-density structures in molecular
clouds, and what role do they play in star formation?

    2. (b)

What generates the observed chemical complexity of molecular gas?

  3. 3.

How do gas, metals, and dust flow into, through, and out of galaxies?

    1. (a)

The production, distribution, and cycling of metals.

    2. (b)

The coupling of small-scale energetic feedback processes to the larger gaseous
reservoir.

  4. 4.

How do the histories of galaxies and their dark matter halos shape their
observable properties?

    1. (a)

Connecting local galaxies to high-redshift galaxies.

    2. (b)

The evolution of morphologies, gas content, kinematics, and chemical
properties of galaxies.

SAFARI-Lite’s high spatial resolution and simultaneous $34-230$ $\mu$m full
spectral coverage, combined with HiRX’s sub-km s-1 spectral resolution and
efficient mapping capabilities using a scanning mirror, make SALTUS the only
observatory that can measure the key diagnostic lines of gas, and
quantitatively address the roles of stellar feedback in the evolution of
regions of massive star formation and of protostellar feedback during the
earliest, deeply embedded phases of star formation, and determine how this
depends on the cluster characteristics and core environment.

###  3.1 Understanding the Role of Feedback in Milky Way Star-forming Regions

---
Figure 3:  Top row: (Left) Part of the Northern (peak 1) outflow lobe in the
BN/KL region as traced in the K band (blue), 1.644$\mu$m [FeII] (green), and
2.122$\mu$m H2 (red), revealing well defined H2 emission with [FeII]
fingertips40. (Right) Outflows driven by the young stellar cluster in the NGC
1333 region. Some 22 outflow lobes are evident in the 4.5$\mu$m IRAC image
(green) and the red and blue shifted CO J=1-0 emission in this 0.5 pc field41.
Bottom row: The bipolar outflow associated with the proto star L1157mm.
(Right) H2O emission in the ground-state ortho line at 179$\mu$m obtained by
Herschel/PACS at a resolution of 13′′, revealing unresolved emission clumps42.
(Middle) A blow up of the blue shifted, southern lobe in the CO J=1-0
transition at a resolution of 3′′. Two bright emission structures, B1 & B2,
are indicated43. (Left) Interferometric studies of molecular emission in the
B1 clump break this structure up into multiple components44. Green contours
trace acetaldehyde and methyl cyanide, respectively, while white is deuterated
formaldehyde and methanol. The clear chemical differentiation of these complex
organic molecules reflects the effects of the interplay of shock sputtering of
icy grains and subsequent chemistry in the warm gas. Beam sizes ($\sim$3′′)
are indicated and comparable to SAFARI-Lite at the longest wavelengths.

A key SALTUS science goal aims to understand the role of feedback in star
forming regions, both in nearby Milky Way regions as well as in nearby spiral
galaxies. It quantifies the kinetic energy and momentum input during the
earliest, deeply embedded phases of star formation, and addresses how this
depends on the cluster characteristics and core environment.

Orion BN/KL and NGC 1333 are the two prototypical Galactic sources of star
formation to be studied that demonstrate SALTUS’s unique capabilities to study
protostellar outflows to the general community. The combination of high
spatial resolution, simultaneous full spectral coverage, and mapping
efficiency make SALTUS the only observatory that can measure the key
diagnostic lines of interstellar gas and quantitatively address the role of
protostellar feedback during the earliest, deeply embedded phases of star
formation and how this depends on the cluster characteristics and core
environment. ALMA can measure low J CO sub-millimeter transitions at high
spatial resolution, but the high J transitions that are key to measuring the
temperature, density, and column density of warm, dense shocked gas occur at
far-IR wavelengths, are only accessible with SALTUS. JWST/MIRI can measure the
pure rotational transitions of H2 in the mid-IR that are good diagnostics of
the shock characteristics as well. However, the small foot-print of MIRI
spectroscopy($<$7.7′′) precludes efficient mapping of regions of extended
emission associated with protostellar outflows. Another probe of these
outflows can be achieved through spectral-mapping in key diagnostic lines of J
and C shocks ([OI], H2O, high J CO) using SAFARI-Lite and HiRX, which probes a
range of cluster characteristics and molecular core properties45, 46, 47.

The BN/KL region in Orion is the nearest site of massive star formation,
presenting an excellent opportunity to study in detail the interaction of
massive protostars with the surrounding molecular core. Scaling the
Herschel/PACS fluxes for peak 148, assuming homogeneous emission, results in
expected line fluxes in the 1-3′′ SALTUS beam of $3\times 10^{-18}$ to
$2\times 10^{-15}$ W m-2 for selected 12CO, H2O and OH lines and the [OI]
63$\mu$m fine-structure line. In 13CO, SALTUS can be expected to detect lines
as high as J=30-29 (E${}_{u}\sim 2500$ K). SAFARI-Lite can map the 5 arcmin2
of peaks 1 and 2 in Orion down to a limiting line flux of $1-3\times 10^{-18}$
W m-2 in 5 hours. This data can be analyzed to determine column density,
preshock density, and shock velocity48 on a $1-3$′′ scale and quantify the
disruption of the Orion Molecular Cloud (OMC) 1 core by this explosion and its
effect on the surrounding star cluster.

NGC 1333 is the most active region of low-mass star formation in the Perseus
molecular cloud, containing a large-membership ($>$100) young clusters, and is
considered the prototypical, nearby (135 pc) cluster-forming region49, 50. NGC
1333 contains many molecular outflows (Figure 3)51, 52, 53 that appear capable
of maintaining turbulence and therefore limiting the rate of star formation54,
55, 56. Typical Herschel/PACS high-J CO and H2O line fluxes of the shock
emission associated with the protostellar outflows in NGC 1333 are $\sim
10-16$ W m-257 and translate into average line fluxes of $6\times 10^{-18}$ W
m-2 in the SALTUS beam. A spectral-spatial study with SAFARI-Lite can map a 1
arcmin2 and detect these lines at S/N ratio 30. Spitzer/IRS H2 studies58 imply
that the shocked gas comprises in total some 5 arcmin2. A fully sampled
$34-230$ $\mu$m spectral-spatial map of the shocked gas in NGC 1333 takes 5
hours with SAFARI-Lite. These data can be analyzed analogously to the Herschel
data using existing shock models and provide on a $\sim$3′′ scale size the
shock characteristics (preshock density, shock temperature, shock velocity)
and quantify the energy and momentum provided by the protostellar outflows for
the NGC 1333 cluster. Combination of these data with ground-based
interferometric maps in complex organic molecules can then address the
influence of shocks on the organic inventory of star forming cores.

###  3.2 Understanding the Role of Feedback in Nearby Galaxies

---
Figure 4:  JWST has revealed new details in the structure of the interstellar
medium (ISM) by observing the dust emission of NGC 628. (Left) The concerted
action of stellar winds and supernovae create superbubbles, typically $<$500
pc across, that drive the evolution of the galactic ecosystem (Image credit:
ESA/Webb, NASA, & CSA, J. Lee and the PHANGS-JWST Team). (Center) SALTUS will
complement these JWST studies, extending high resolution observations of the
ISM into the FIR. (Image credit: 24). (Right) As the simulated SAFARI-Lite
image illustrates, SALTUS will simultaneously measure key diagnostic atomic
fine-structure lines like the three simulated here, resolving these bubbles
and quantifying the physical conditions and energetics of this feedback.

The second part of this science case bridges “ground-truth” observations of
feedback on the scale of individual Milky Way star-forming regions to the
spatially unresolved view of galaxy evolution throughout the distant universe.
This part of the science investigation bridges the gap by adding the detailed
local-universe perspective on key Decadal Survey10 science areas.

Energetic feedback from star formation processes, the lives and deaths of
stars, and nuclear black holes (or active galactic nuclei, AGN) are critical
factors in the growth and evolution of galaxies and the gas that surrounds
them. The effects of feedback are multiscale, ranging from individual stars
and clusters to galaxy-scale outflows extending several kiloparsecs into the
circumgalactic medium. However, the injection sites of feedback are small-
scale, originating from individual supernovae, star clusters, and near the AGN
itself (Figure 4).

Spectral line strengths and ratios (e.g., [SiII] 35$\mu$m, [OI] 63, 145$\mu$m,
[OIII] 52, 88$\mu$m, [NII] 122, 205$\mu$m) are used as diagnostics of
radiation field, temperature, and/or density to characterize shocks and feed-
back in the ISM, especially when coupled with modeling codes such as CLOUDY59.
The [CII] line at 158$\mu$m is a tracer of the neutral ISM, photodissociation
regions, and star formation60, 61, 62. [CII] and [OI] 63$\mu$m are major
cooling channels for the ISM. The [OIII] and [NII] lines are tracers of
ionized gas in HII regions and the diffuse ISM, respectively. Since FIR lines
largely govern the bulk heating and cooling of the ISM, they are key to
understanding energy balance in the ISM, especially in regions of active
feedback.

Figure 4 gives an example in a nearby, star-forming galaxy M 74 (The Phantom
Galaxy or NGC 628). While new observations from JWST have resolved the galaxy
and sites of stellar feedback in stunning detail (Figure 4)37, 23, 24, the
physics of the photodissociation regions surrounding these supernova bubbles
is unknown without FIR diagnostics. To understand how ISM heating and cooling
vary under such conditions requires high spatial resolution observations of
key FIR diagnostics. SAFARI-Lite enables such detailed studies of the effects
of stellar and AGN feedback across a suite of nearby galaxies. The
capabilities are highlighted in Figure 4 (right), showing the anticipated
[CII] and [NII] 205$\mu$m emission from M 74 at SAFARI-Lite resolution. The
[CII] emission is estimated from the continuum-subtracted 3.3$\mu$m PAH
feature63, and the [NII] emission is based on H$\alpha$64. A 100$\sigma$
detection of the [CII] 158$\mu$m line (to enable robust detection of weaker
lines and continuum) over the full 9′$\times$9′ extent of M 74 requires 12
hours.

HiRX enables high spectral resolution (R$>10^{6}$, $\Delta$v$<$1 km/s)
observations of strong emission lines (e.g., [CII] 158$\mu$m, [NII] 205$\mu$m,
[OI] 145$\mu$m, H2O, several high-J CO lines; Figure 2). The precision
kinematics yield robust measurements of energy and momentum injection from
feedback and shocks on scales ranging from star clusters to large-scale
outflows65, 66. In M 74 specifically (Figure 4), the expansion of the “Phantom
Void” is $15-50$ km s-1. HiRX will be the only instrument capable of spatially
and spectrally resolving feedback-driven bubbles, which are expected to be
commonplace in star-forming galaxies23, 24.

###  3.3 Example Community Science: Stellar Feedback in Regions of Massive
Star Formation

---
Figure 5:  Simulated SALTUS image at 2.5′′ angular resolution (middle) of the
[CII] 158$\mu$m emission in NGC 6611 (Pillars of Creation) is similar to the
JWST NIRCam image (left) and compared to the SOFIA-created map67 (right).
SAFARI-Lite can map this 10 arcmin2 region in 10 hours and simultaneously
provide maps in all diagnostic lines and photo-dissociation regions (PDRs) and
HII regions in our galaxy and the local group probing the physical environment
produced by radiation feedback of massive stars and its link to stellar
clusters and its molecular core.

This topic aims to understand the role of stellar feedback in regions of
massive star formation: How much kinetic energy is contributed to the ISM, how
does this depend on the characteristics of the region, and, connected to this,
how does the surrounding medium react? This objective is addressed by
spectral-spatial mapping of regions of massive star formation in the dominant
far-IR cooling lines of PDR gas using both the SAFARI-Lite and HiRX
instruments (c.f., Pillars of Creation in Figure 5). The sample consists of 25
regions with typical sizes of 25 arcmin2, covering a range in stellar cluster
properties (single stars to small clusters to superstar clusters), GLIMPSE
bubble morphology (ring-like, bi-or multi-polar, complex), environment
(isolated star forming region, galactic mini starburst, nuclear starburst),
and cluster age (0.1 to 5 Myr).

SAFARI-Lite will provide full spectra over the 34-230$\mu$m range, containing
the key diagnostic atomic fine-structure transitions. SOFIA/upGREAT studies of
the [CII] 158$\mu$m emission of regions of massive star formation have
demonstrated the feasibility of this technique68, 69. Together these two data
sets quantify the kinetic energy of the expanding shell as well as the
thermal, turbulent, and radiation pressures on the shell that can be directly
compared to radiative and mechanical energy inputs of the stellar cluster.

###  3.4 Example Community Science: Putting Early-Universe Feedback in Context
with Local Analogs of the First Galaxies

The first year of JWST observations revealed that early generations of
galaxies were typically much lower in mass and significantly less metal-
enriched compared to the massive star-forming spiral galaxies that dominate
star formation today. Consequently, their UV radiation fields were harder70,
which impacted the gas ionization state and cooling rate. The shallow
gravitational potential wells also enhanced the effects of supernova-driven
feedback since single supernovae could eject up to $\sim$95% of the heavy
elements formed during the star’s lifetime into the circumgalactic medium71,
72. While ALMA now allows the key far-IR fine structure diagnostic lines
(e.g., [CII], [OIII]) to be detected at high redshifts, these lines still take
several hours to detect even in the most massive and UV-luminous reionization-
era galaxies73 and are generally out of reach for the more typical galaxies
thought to drive cosmic reionization.

Thanks to community efforts, many nearby galaxy populations have been
identified bearing similarities to UV-bright reionization-era galaxies in
terms of their mass, metallicity, and harsh UV radiation fields. Though rare
locally and largely unknown during the time of Herschel, these low-redshift
galaxies are plausible analogs to early star-forming galaxies. Such local low-
metallicity dwarf galaxies are natural extensions to the SALTUS low-redshift
GTO efforts to measure and resolve the injection of feedback energy on small
spatial scales far beyond the capabilities of Herschel74 or a 1m-class FIR
mission. With the same observables as the GTO program of star-forming spirals
and starbursts, and the most commonly detected lines found with ALMA at high
redshifts, SALTUS enables $\sim$200 pc-scale maps of the FIR fine structure
lines and underlying dust continuum in low-redshift analogs of the first
generation of galaxies.

###  3.5 Example Community Science: Spectral Line Survey

The benefits of performing high-frequency spectral line surveys were
recognized and exploited by Herschel HIFI, which resulted in the first
reported detections of SH+, HCl+, H2O+, and H2CL+ 75, 76, 77. The formation of
these small hydrides typically represents the first step in gas phase
chemistry routes toward molecular complexity in space. However, the spectral
surveys with HIFI were severely limited by sensitivity. SALTUS’s increased
aperture results in a 16$\times$ increased sensitivity for these unresolved
sources and can be expected to lead to an enhanced line density and discovery
space. Moreover, the wavelength coverage of SALTUS bridges the gap between
spectral line surveys to be carried out with JWST/MIRI and ALMA78.

##  4 Conclusions

Unlocking the details of ISM heating and cooling, feedback, and star-formation
as a function of environment are keys to understanding our Cosmic Ecosystem10
and rely on measurements that can only be made in the FIR. While any future
FIR facility will offer transformative gains over previous observatories at
this wavelength, only SALTUS will enable the spatially-resolved studies in
nearby galaxies to probe a wider range of conditions and to inform numerical
simulations. Priority Milky Way measurements include measuring PDRs and
feedback in prototypical high- and low-mass star forming regions at
unprecedented resolution and sensitivity, bridging the gaps between studies of
star and planet formation and the ISM. In nearby galaxies, the priority is to
measure PDRs and feedback in well-characterized prototypes (e.g., M 74) across
a range of environments. SALTUS’ transformative increase in spatial
resolution, spectral coverage, and sensitivity at these wavelengths ensure
that it will be responsive to the needs of and bridge gaps between the
Galactic and nearby extragalactic communities of the 2030s. No other FIR
facility will provide such seamless continuity in our understanding of the ISM
after a decade of JWST observations and ALMA/ALMA-2030.

### Disclosures

The authors have no relevant financial interests in the manuscript nor do they
have any conflicts of interest to disclose.

### Code and Data Availability

No code or data were used in the preparation of this manuscript.

### Acknowledgments

R.C.L. acknowledges support for this work provided by a National Science
Foundation (NSF) Astronomy and Astrophysics Postdoctoral Fellowship under
award AST-2102625. This work has made use of NASA’s Astrophysics Data System
Bibliographic Services. This work has made use of the NASA/IPAC Extragalactic
Database (NED), which is funded by NASA and operated by the California
Institute of Technology.

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Rebecca C. Levy is a NSF Astronomy & Astrophysics Postdoctoral Fellow at the
University of Arizona. She received her BS in Astronomy and Physics from the
University of Arizona in 2015, and her MS and PhD degrees in Astronomy from
the University of Maryland, College Park in 2017 and 2021, respectively. She
has authored more than 40 refereed journal papers. Her research interests
include studying extreme star formation, star clusters, stellar feedback, and
the interstellar medium in nearby galaxies using multiwavelength ground- and
space-based observations.

Alexander Tielens is a professor of astronomy in the Astronomy Department of
the University of Maryland, College Park. He got his masters and PhD in
astronomy from Leiden University in 1982. He has authored over 500 papers in
refereed journals and written two textbooks on the interstellar medium. His
scientific interests center on the physics and chemistry of the interstellar
medium, in particular in regions of star and planet formation, including the
characteristics and origin and evolution of interstellar polycyclic aromatic
hydrocarbon molecules and dust grains. He has published extensively on the
radiative and mechanical feedback of massive stars on their environment;
especially, the observational and theoretical characteristics of
photodissociation regions.

Justin Spilker is an assistant professor in the Department of Physics and
Astronomy and the Mitchell Institute of Texas A&M University.

Daniel Marrone is a professor of astronomy at the University of Arizona.

Desika Narayanan is an associate professor of astronomy at the University of
Florida, and an affiliate of the Cosmic Dawn Center in Copenhagen. He mainly
studies the physics of the interstellar medium in galaxies near and far via
large scale numerical simulations of galaxy evolution. He earned his BS from
the University of Florida in 2003, his PhD from the University of Arizona in
2007, and was the CfA fellow at the Harvard-Smithsonian Center for
Astrophysics, as well as the Bok Fellow at the University of Arizona. He was a
professor at Haverford College from 2014-2017, prior to returning to the
University of Florida.

Christopher K. Walker is a professor of astronomy at the University of
Arizona, and the principal investigator of the SALTUS mission concept.

Biographies and photographs of the other authors are not available.

###### List of Figures

  1. 1 The role of feedback and the lifecycle of galaxies. Stars form inside dense cloud. The mechanical and radiative energy injected during their evolution from the protostellar to the main sequence to the supernova phase will heat interstellar gas, inject turbulence into the medium, disrupt molecular clouds, and create the various phases of the ISM. During the asymptotic giant branch phase and the supernova phase, newly synthesized elements will be ejected, slowly enriching the ISM. The concerted action of massive stars in rich OB associations will drive the formation of chimneys that vent enriched gas into the lower halo and even the intergalactic medium. Halo gas will condense in clouds that rain back onto the galactic plane. The concerted action of these processes drives the evolution of galaxies. Figure taken from 10.
  2. 2 An example of the wavelength coverage of SALTUS compared to a spectrum of the archetypal nearby starburst galaxy M 82 (black; from Spitzer/IRS33, ISO/LWS34, and Herschel/SPIRE35). The SAFARI-Lite simultaneous coverage is shown in blue and the HiRX bands are shown in purple. The coverage of JWST/MIRI (light pink) and the ALMA bands (dark pink) are plotted for reference. Key diagnostic lines are shown, tracing the ionized (red), neutral (yellow), molecular (blue), ionized/neutral (orange), and ionized/neutral/molecular (green) ISM phases; this list is not exhaustive. With SALTUS, we will measure spectra of nearby galaxies at substantially higher spatial resolution, spectral resolution, and sensitivity compared to previous instruments, opening a fundamentally new window into the study of the ISM in the local Universe.
  3. 3  Top row: (Left) Part of the Northern (peak 1) outflow lobe in the BN/KL region as traced in the K band (blue), 1.644$\mu$m [FeII] (green), and 2.122$\mu$m H2 (red), revealing well defined H2 emission with [FeII] fingertips40. (Right) Outflows driven by the young stellar cluster in the NGC 1333 region. Some 22 outflow lobes are evident in the 4.5$\mu$m IRAC image (green) and the red and blue shifted CO J=1-0 emission in this 0.5 pc field41. Bottom row: The bipolar outflow associated with the proto star L1157mm. (Right) H2O emission in the ground-state ortho line at 179$\mu$m obtained by Herschel/PACS at a resolution of 13′′, revealing unresolved emission clumps42. (Middle) A blow up of the blue shifted, southern lobe in the CO J=1-0 transition at a resolution of 3′′. Two bright emission structures, B1 & B2, are indicated43. (Left) Interferometric studies of molecular emission in the B1 clump break this structure up into multiple components44. Green contours trace acetaldehyde and methyl cyanide, respectively, while white is deuterated formaldehyde and methanol. The clear chemical differentiation of these complex organic molecules reflects the effects of the interplay of shock sputtering of icy grains and subsequent chemistry in the warm gas. Beam sizes ($\sim$3′′) are indicated and comparable to SAFARI-Lite at the longest wavelengths.
  4. 4  JWST has revealed new details in the structure of the interstellar medium (ISM) by observing the dust emission of NGC 628. (Left) The concerted action of stellar winds and supernovae create superbubbles, typically $<$500 pc across, that drive the evolution of the galactic ecosystem (Image credit: ESA/Webb, NASA, & CSA, J. Lee and the PHANGS-JWST Team). (Center) SALTUS will complement these JWST studies, extending high resolution observations of the ISM into the FIR. (Image credit: 24). (Right) As the simulated SAFARI-Lite image illustrates, SALTUS will simultaneously measure key diagnostic atomic fine-structure lines like the three simulated here, resolving these bubbles and quantifying the physical conditions and energetics of this feedback.
  5. 5  Simulated SALTUS image at 2.5′′ angular resolution (middle) of the [CII] 158$\mu$m emission in NGC 6611 (Pillars of Creation) is similar to the JWST NIRCam image (left) and compared to the SOFIA-created map67 (right). SAFARI-Lite can map this 10 arcmin2 region in 10 hours and simultaneously provide maps in all diagnostic lines and photo-dissociation regions (PDRs) and HII regions in our galaxy and the local group probing the physical environment produced by radiation feedback of massive stars and its link to stellar clusters and its molecular core.