# Atacama Large Aperture Submillimeter Telescope
(AtLAST) Science: Surveying the distant Universe

Eelco van Kampen  European Southern Observatory, Karl-Schwarzschild-Str. 2,
85748 Garching bei München, Germany  Tom Bakx  Department of Space, Earth, &
Environment, Chalmers University of Technology, Chalmersplatsen 4, Gothenburg
SE-412 96, Sweden  Carlos De Breuck  European Southern Observatory, Karl-
Schwarzschild-Str. 2, 85748 Garching bei München, Germany  Chian-Chou Chen
Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Sec.
4, Roosevelt Road, Taipei 10617, Taiwan  Helmut Dannerbauer  Instituto de
Astrofísica de Canarias (IAC), E-38205 La Laguna, Tenerife, Spain  Universidad
de La Laguna, Dpto. Astrofísica, E-38206 La Laguna, Tenerife, Spain  Benjamin
Magnelli  Université Paris-Saclay, Université Paris-Cité, CEA, CNRS, AIM,
91191 Gif-sur-Yvette, France  Francisco Miguel Montenegro-Montes  European
Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München,
Germany  Universidad Complutense de Madrid, Departamento de Física de la
Tierra y Astrofísica e Instituto de Física de Partículas y del Cosmos
(IPARCOS), Ciudad Universitaria, 28040 Madrid, Spain  Teppei Okumura  Academia
Sinica Institute of Astronomy and Astrophysics (ASIAA), No. 1, Sec. 4,
Roosevelt Road, Taipei 10617, Taiwan  Sy-Yun Pu  Graduate Institute of
Astronomy, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road,
Hsinchu 30013, Taiwan  Academia Sinica Institute of Astronomy and Astrophysics
(ASIAA), No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan  Matus Rybak
Faculty of Electrical Engineering, Mathematics and Computer Science, Mekelweg
4, 2628 CD Delft, the Netherlands  Leiden Observatory, Leiden University,
Niels Bohrweg 2, 2333 CA Leiden, the Netherlands  SRON - Netherlands Institute
for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands  Amélie
Saintonge  Department of Physics and Astronomy, University College London,
Gower Street, London WC1E 6BT, UK  Max-Planck-Institut für Radioastronomie
(MPIfR), Auf dem Hügel 69, D-53121 Bonn, Germany  Claudia Cicone  Institute of
Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, 0315
Oslo, Norway
Evanthia Hatziminaoglou  European Southern Observatory, Karl-Schwarzschild-
Str. 2, 85748 Garching bei München, Germany  Instituto de Astrofísica de
Canarias (IAC), E-38205 La Laguna, Tenerife, Spain  Universidad de La Laguna,
Dpto. Astrofísica, E-38206 La Laguna, Tenerife, Spain  Juliëtte Hilhorst
Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, the
Netherlands  Department of Astronomy, Yale University, New Haven, CT 06511,
USA  Pamela Klaassen  UK Astronomy Technology Centre, Royal Observatory
Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK  Minju Lee  Cosmic Dawn
Center (DAWN), Denmark  DTU-Space, Technical University of Denmark, Elektrovej
327, DK2800 Kgs. Lyngby, Denmark
Christopher C. Lovell  Institute of Cosmology and Gravitation, University of
Portsmouth, Burnaby Road, Portsmouth, PO1 3FX, UK  Andreas Lundgren  Aix
Marseille Univ, CNRS, CNES, LAM, Marseille, France  Luca Di Mascolo
Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur,
CNRS, Blvd de l’Observatoire, CS 34229, 06304 Nice cedex 4, France  Astronomy
Unit, Department of Physics, University of Trieste, via Tiepolo 11, Trieste
34131, Italy  INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11,
Trieste 34131, Italy  IFPU – Institute for Fundamental Physics of the
Universe, Via Beirut 2, 34014 Trieste, Italy  Tony Mroczkowski  European
Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München,
Germany  Laura Sommovigo  Center for Computational Astrophysics, Flatiron
Institute, 162 5th Avenue, New York, NY 10010, USA  Mark Booth  UK Astronomy
Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9
3HJ, UK  Martin A. Cordiner  Astrochemistry Laboratory, Code 691, NASA Goddard
Space Flight Center, Greenbelt, MD 20771, USA.  Rob Ivison  European Southern
Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
School of Cosmic Physics, Dublin Institute for Advanced Studies, 31
Fitzwilliam Place, Dublin D02 XF86, Ireland  Institute for Astronomy,
University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ,
UK  ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO
3D)  Doug Johnstone  NRC Herzberg Astronomy and Astrophysics, 5071 West
Saanich Rd, Victoria, BC, V9E 2E7, Canada  Department of Physics and
Astronomy, University of Victoria, Victoria, BC, V8P 5C2, Canada  Daizhong Liu
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1,
Garching, Bayern, D-85748, Germany  Purple Mountain Observatory, Chinese
Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China  Thomas J.
Maccarone  Department of Physics & Astronomy, Texas Tech University, Box
41051, Lubbock TX, 79409-1051, USA  Matthew Smith  School of Physics &
Astronomy, Cardiff University, The Parade, Cardiff CF24 3AA, UK  Alexander E.
Thelen  Division of Geological and Planetary Sciences, California Institute of
Technology, Pasadena, CA 91125, USA.  Sven Wedemeyer  Institute of Theoretical
Astrophysics, University of Oslo, P.O. Box 1029, Blindern, 0315 Oslo, Norway
Rosseland Centre for Solar Physics, University of Oslo, Postboks 1029
Blindern, N-0315 Oslo, Norway

###### Abstract

During the most active period of star formation in galaxies, which occurs in
the redshift range $1<z<3$, strong bursts of star formation result in
significant quantities of dust, which obscures new stars being formed as their
UV/optical light is absorbed and then re-emitted in the infrared, which
redshifts into the mm/sub-mm bands for these early times. To get a complete
picture of the high-$z$ galaxy population, we need to survey a large patch of
the sky in the sub-mm with sufficient angular resolution to resolve all
galaxies, but we also need the depth to fully sample their cosmic evolution,
and therefore obtain their redshifts using direct mm spectroscopy with a very
wide frequency coverage.

This requires a large single-dish sub-mm telescope with fast mapping speeds at
high sensitivity and angular resolution, a large bandwidth with good spectral
resolution and multiplex spectroscopic capabilities. The proposed 50-m Atacama
Large Aperture Submillimeter Telescope (AtLAST) will deliver these
specifications. We discuss how AtLAST allows us to study the whole population
of high-z galaxies, including the dusty star-forming ones which can only be
detected and studied in the sub-mm, and obtain a wealth of information for
each of these up to $z\sim 7$: gas content, cooling budget, star formation
rate, dust mass, and dust temperature.

We present worked examples of surveys that AtLAST can perform, both deep and
wide, and also focused on galaxies in proto-clusters. In addition we show how
such surveys with AtLAST can measure the growth rate f8 and the Hubble
constant with high accuracy, and demonstrate the power of the line-intensity
mapping method in the mm/sub-mm wavebands to constrain the cosmic expansion
history at high redshifts, as good examples of what can uniquely be done by
AtLAST in this research field.

## Keywords

cosmology — galaxy surveys — galaxy formation —
sub-mm galaxies — cluster galaxies

## Plain language summary

Galaxies come in a wide variety of shapes, sizes, and colours, despite all of
them having originated from similar initial conditions in the early Universe.
Understanding this diversity by tracing back the evolutionary pathways of
different types of galaxies is a major endeavour in modern astrophysics.
Galaxies build their stellar mass over time by converting gas into stars
through various episodes of star formation. Understanding exactly when, where,
and how this star formation process is triggered or suppressed is therefore a
crucial question to answer.

Current observations reveal that the Universe was at its most active (in terms
of star formation rate per unit volume) in the distant past, about 10 billion
years ago. By measuring the amount of gas and dust in galaxies at that epoch,
we also know that the reason for this very high star formation activity is
large reservoirs of gas (the fuel for star formation) and the higher
efficiency of galaxies at converting their gas into stars. However, recent
work also reveals that we are missing significant numbers of distant actively
star-forming galaxies in current samples because these are obscured by dust,
and therefore our picture is still very incomplete.

In this paper, we explore how a new proposed telescope, the Atacama Large
Aperture Submillimeter Telescope (AtLAST)111http://atlast-telescope.org/, can
provide us with the very important missing pieces of this puzzle. AtLAST will
allow us to map large areas of the sky at unprecedented depth, resolution and
multiplex spectroscopic capabilities. This telecope would provide us with a
complete, homogeneous and unbiased picture of the star-forming galaxy
population in the early Universe. Not only will we be able to discover these
galaxies, but also measure their distances, the composition of their gas and
dust content, and the rate at which they convert gas into stars.

##  Introduction

In the distant Universe, the star formation rate density in galaxies is
highest in the redshift range $1<z<3$ (eg.Madau & Dickinson 2014), which
results in a fair amount of astrophysical dust and gas in these galaxies. A
similar trend is observed in the cold-gas content of galaxies - as traced by
emission lines and cold dust, which is seen to peak at a similar redshift
range (e.g. Tacconi et al. 2020). The dust obscures new stars being formed,
because their UV/optical light is absorbed and then re-emitted in the infrared
(IR, eg. Salim & Narayanan 2020 for an excellent review on the physical
mechanisms), contributing to the so-called cosmic far-IR background (CIB).
This accounts for about half of the energy density from star formation,
integrated over the history of the Universe (Dole et al., 2006). The infrared
photons emitted in the rest-frame of these galaxies get redshifted into the
submillimeter and millimeter ((sub-)mm) observed frame, and the negative
K-correction at this wavelength regime enables galaxies to appear roughly
constant in the observed flux densities at $z\sim 1-10$ (Blain et al., 2002).
This means that the (sub-)mm is an essential wavelength range for studying
high-$z$ galaxies in order to understand their star formation and growth.
Since high-$z$ star forming galaxies are best observed in the (sub-)mm, they
are often called ’sub-mm galaxies’ (SMGs), although lately the physically-
motivated denomination of ‘dusty star forming galaxies’ (DSFGs) is preferred.
The study of DSFGs now forms a rich research field: for reviews on its history
see Carilli & Walter (2013), Casey et al. (2014), Combes (2018). In the 25
years since the discovery of DSFGs a number of surveys have targeted the
(dust) continuum and spectral line emission from these high-redshift galaxies.

Wide-field deep continuum surveys with many-pixel bolometer detectors on
single-dish telescopes have been efficient in discovering DSFGs out to the
epoch of re-ionisation ($z\approx 6$). From the ground, the South Pole
Telescope (SPT) conducted a $\sim$2500 deg2 shallow survey at 1.4 mm and 2 mm,
uncovering almost a hundred (mostly gravitationally lensed) dusty galaxies
(Reuter et al. 2020, Everett et al. 2020 and references therein). A similar
survey has been completed by the 6-m Atacama Cosmology Telescope (ACT) (Gralla
et al., 2020). At 850 m, several deg2 have been surveyed by the LABOCA camera
on APEX (Weiß et al., 2009) and the SCUBA-2 camera on JCMT (Geach et al.,
2017). Due to their low angular resolution, these surveys are confusion-
limited at the mJy level, i.e., faint sources start overlapping. Current
continuum survey facilities include the NIKA-2 camera on the 30-m IRAM
telescope, A-MKID 350/850-m camera on APEX, and the TolTEC camera on the 50-m
LMT.

In space, Herschel mapped up to $1270$ deg2 at 250 - 500 m, revealing 1.7
million dusty galaxies (with multiple detections), as collected in the
Herschel Extragalactic Legacy Project (HELP, Shirley et al. 2021), which
notably includes the Herschel Multi-tiered Extragalactic Survey (HerMES,
Oliver et al. 2012) and the Herschel Atlas survey (H-ATLAS, Eales et al.
2010). About a dozen extremely bright / highly magnified high-$z$ galaxies
were detected in the all-sky (but relatively shallow) continuum imaging with
the Planck telescope (Lammers et al., 2022). The redshift distribution depends
on the selection wavelength (e.g. Béthermin et al. 2015), so there is a clear
need for a wide frequency range in observations to enable a complete census of
dusty galaxies from cosmic noon to cosmic dawn.

ALMA has carried out several “wide-field” continuum surveys, starting with the
1.3-mm survey of the HUDF (Dunlop et al., 2017, $\approx$4.5 arcmin2). The
currently most extensive interferometric survey is the 2-mm MORA survey (Casey
et al., 2021, 184 arcmin2), with an extension currently being produced. In
contrast to the continuum mapping, blind spectral-line surveys have been
limited to interferometers - e.g., ASPECS (Walter et al., 2016), ALMACAL
(Klitsch et al., 2019), and ALCS (Fujimoto et al., 2023) on ALMA, and HDF-N
survey with NOEMA (Boogaard et al., 2023, 8 arcmin2). This is because current
spectroscopic instruments on single-dish telescopes are often limited to
single-pixel designs, and the few exceptions have a maximum of up to ten
spatial elements. Instead, the interferometers act as an “integral field unit”
(IFU) within their limited field-of-view of interferometers, that cover an
area roughly equal to a single pixel element of a single-dish telescope.
Multi-object spectroscopy - the ability to obtain spectra of multiple objects
in the field-of-view of the telescope, common at optical / near-IR wavelengths
- is virtually non-existent in the sub-mm wavebands. As such, these surveys
are inherently restricted to pencil-beam observations due to the low mapping
speeds. ALMA Large Programs such as ALPINE (Faisst et al., 2020, Béthermin et
al., 2020) and REBELS (Bouwens et al., 2022, Inami et al., 2022) have provided
the first statistical sample of $4<z<7$ (up to 7) dust continuum emitting
galaxies (albeit UV-selected, not DSFGs), but these were targeted surveys, not
blind ones. The contribution to the star-formation rate density of these more
’normal’ (but still massive: $M_{*}>10^{9},1^{10}M_{\odot}$ for ALPINE/REBELS)
dusty galaxies seems to be far from negligible ($>30$% at $z$=7, see e.g.
Algera et al. 2023, Barrufet et al. 2023). There are many uncertainties due to
selection bias, and lack of multiple ALMA observations for most of the
targets, but AtLAST will have the sensitivity to detect these sources.

To date, the science driven by sub-mm observations has focused on a
combination of large-area continuum observations from both ground- and space-
based observations, while spectroscopic observations with a sufficient spatial
resolution to resolve these galaxies are limited to small areas. This bimodal
approach has left a discovery space that can only be addressed by a telescope
with a large primary mirror, exploiting recent advances in instrumentation to
provide Integral Field Unit (IFU) capabilities on the scale of individual
galaxies. AtLAST is conceived to realise this by combining a large throughput
single dish facility with a powerful instrumentation suite. How will AtLAST
outperform current facilities? High angular resolution (lowering confusion
noise); large collecting area and large focal plane - high survey speed (see
e.g. Klaassen et al., 2020, Ramasawmy et al., 2022, Mroczkowski et al., 2023,
2024). For example, the expected sky area covered by AtLAST with a single
pointing will be about 200 times larger than the LMT. In addition, the
photometric confusion noise at 350 m will be a factor of $>10000\times$ lower
than that of 6-m telescopes like ACT and CCAT-p ($<0.2\leavevmode\nobreak\
\mu$Jy for AtLAST vs. 2600 Jy for ACT and CCAT-p). The improvement in angular
resolution provided by AtLAST is demonstrated clearly in Figure 1, which
compares what can be achieved by current 6-m telescopes like ACT to what
AtLAST can do.

Here we describe the compelling science that requires a new facility in order
to be achieved - specifically AtLAST, a 50m single dish sub-mm telescope with
the capabilities listed above (incl. high mapping efficiency). Such an
observatory would enable a large-area spectroscopic survey, expose baryon
acoustic oscillations, probe the unresolved power-spectrum of galaxies (i.e.,
line intensity mapping) and reveal the largest coherent structures in the
Universe towards the early Universe.

Figure 1: A simulated 277-GHz continuum map with the 6-m ACT telescope (left)
and AtLAST (right) _without_ instrument noise. The ACT map is limited by
confusion noise (notice the seemingly overlapping faint sources); thanks to
its 50-m dish, AtLAST will be able to resolve much fainter (up to four orders
of magnitude) individual sources. These mock images are based on simulated
galaxy catalogues from Lagos et al. (2020).

##  Science Goals

In the following subsections we outline the high-$z$ science goals for AtLAST,
focusing on the overall high-$z$ galaxy population as well on galaxies in
proto-clusters. We also discuss how to extract cosmological parameters from
the large survey, and the novel line-intensity mapping technique. We refer to
Lee et al. (in prep.) and Di Mascolo et al. (in prep.) for companion AtLAST
case studies focused on emission line probes of the cold circumgalactic medium
(CGM) of galaxies and on probing the Intra-Cluster Medium (ICM) using the full
Sunyaev-Zeldovich (SZ) spectrum to understand the thermal history of the
Universe, respectively. Additional AtLAST science cases outside the research
fields of the distant Universe and cosmology are presented by Booth et al. (in
prep), Cordiner et al. (in prep), Klaassen et al. (in prep), Liu et al. (in
prep), Orlowski-Scherer et al. (in prep), and Wedemeyer et al. (in prep). For
the purpose of this paper, “high-$z$” refers to $z>1$.

###  A large homogeneous galaxy survey in the distant Universe

The integrated spectral energy distribution of the CIB is nowadays relatively
well constrained thanks to accumulated observations over the past decades from
ground and space facilities, in particular close to the peak of thermal IR
emission around 150 m in the rest-frame. Still, at sub-mm wavelengths, where
the contributions from high-redshift galaxies are expected to dominate, a fair
fraction of this emission remains unresolved and only the brightest population
of DSFGs has been identified and studied in certain detail. Many of those have
been identified in blind large area-surveys, their redshift determined thanks
to CO spectroscopy in the mm-regime, and then followed up and studied in
detail with interferometers like ALMA, as in Reuter et al. (2020).

Little is known observationally about the less extreme population of normal
dusty galaxies, accounting for the bulk of the objects contributing to the CIB
at these wavelengths. Studying this population is crucial to progressing our
understanding of numerous open questions like the co-evolution of star
formation and black-hole growth, as most high-z star formation occurs in
galaxies deeply embedded in dust (see Carraro et al. 2020, Mountrichas &
Shankar 2023). Or the evolution of the dust properties over cosmic time, which
is under intense debate (see e.g. Sommovigo et al. 2022, Dayal et al. 2022,
Drew & Casey 2022, Hirashita & Il’in 2022, Di Cesare et al. 2023). Till now,
the study of these less extreme sub-mm galaxies has been relying partially on
extrapolation of properties of MIR- or radio-selected galaxies with emission
in FIR/sub-mm. I Another approach has been studying the physical properties of
some of the brightest DSFGs which are associated with lensed systems. Both
approaches are severely biased against the brightest end of the population.

In order to determine accurately the number counts and the redshift
distribution of the population of normal dust star-forming galaxies, it is
crucial to conduct deep and unbiased surveys (continuum and spectroscopic)
over large areas with high enough spatial resolution, as the ones we propose
to do with AtLAST. Such large, homogeneous survey of DSFGs allows for an
angular clustering analysis, a determination of the (photometric) redshift
distribution, number count estimates, and many other statistical properties.
It will provide a high-$z$ counterpart to extensively studied large galaxy
samples at low and intermediate redshifts.

Perhaps one of AtLAST’s most important contributions to this field will be to
study the role of the environment (voids, filaments, groups, clusters) and the
way the evolution of DSFGs varies as a function of this environment. This is
particularly complementary in the era of Euclid, LSST, Roman and SKA.

The relatively strong negative K-correction in the sub-mm for high-redshift
dusty galaxies has been extensively exploited to efficiently: detect DSFGs
(either lensed or unlensed) out to high redshifts, and study them in various
amounts of detail, depending on the sensitivity and angular resolution of the
sub-mm observatory used for the study. Currently, the ALMA interferometer has
the best of both, it is not, however, able to study large samples due to its
small field of view - ALMA is not a survey instrument. To study statistical
populations of high-$z$ galaxies, and environmental effects on their
evolution, a complementary, dedicated type of instrument is required, with a
high survey speed, low confusion noise limits, and sufficient sensitivity,
which is what AtLAST will deliver.

We aim for a comprehensive multi-band imaging survey, uniquely mapping large
parts of the sky to specifically target high-$z$ galaxies and map their
distribution (noting that this could be done in combination with imaging
surveys for other science cases). Using a multi-chroic camera, several of the
bands can be observed simultaneously, allowing for accurate spectral slope
determinations and photometric redshifts (especially in combination with
complementary data available at other wavebands), as the observing conditions
will be identical for each of the bands. This will provide a rich catalogue of
sources to follow-up with ALMA, JWST, or ELT, but more importantly, yield a
large homogeneous sample of galaxies in the early Universe.

Wide-field “blind” spectral line surveys with AtLAST will be crucial for
mapping the population of “normal” star-forming galaxies across the cosmic
history. These are often too faint in dust emission to be detected in
continuum surveys. However, as suggested by recent predictions from
simulations (e.g., Lagos et al. 2020), while the cosmic star-forming activity
is dominated by sub-mm bright galaxies ($S_{\mathrm{350\,GHz}}\geq$ 1 mJy),
the gas budget of the Universe is dominated by sub-mm faint galaxies
(quantitatively: $\approx$75% of the gas budget at $z=2$, and $\geq$90% at
$z\geq 3$). Deep, wide-bandwidth spectroscopic surveys of CO and [CII]
emission with AtLAST will be critical for mapping the evolution of cold-gas
content across the cosmic history. Especially [CII] is a very valuable tracer,
physically. It correlates well with the star formation rate (De Looze et al.,
2014), is a valid tracer of the bulk of the gas mass (see Wolfire et al. 2022
for a recent review, and Zanella et al. 2018 for a high-z empirical study),
and it is one of the brightest FIR lines out to high-z, as demonstrated by the
detection rate of recent ALMA Large Programs, e.g. ALPINE (Faisst et al.,
2020, Béthermin et al., 2020) and REBELS (Bouwens et al., 2022, Inami et al.,
2022).

In the following we explore two worked examples of surveys that AtLAST will
make possible. Often a ’wedding-cake" approach is followed, where several
surveys are planned with different angular sizes and sensitivity limits, each
forming a layer of an imaginary wedding cake. Our two worked examples form the
two extremes: the bottom and top layers of the cake, but we will certainly
consider the other layers as well, although these could hit the confusion
limit for the longer wavelengths if the survey area is too small. An
additional pointed survey of galaxy clusters is discussed in Section 2.4.1.
With respect to the continuum survey, we list a likely set of frequency bands
with associated sensitivities and beam sizes for AtLAST in the companion
high-$z$ paper by Di Mascolo et al. (in prep.). These are optimized for a
range of AtLAST science goals, including the ones presented in this paper.

####  A wide continuum survey

Figure 2: The mean (top panel) and dispersion (bottom panel) of log10(LIR-BB /
LIR-True) and log10(Mdust-BB / Mdust-true) for mock observations of the SED
library of Draine & Li (2007).

To estimate what can actually be achieved with AtLAST, we consider a 1000 deg2
mock galaxy survey as could be obtained in 1000 hours of observing time in the
continuum in two or more bands. Such a continuum survey allows one to infer
two important physical properties of galaxies: their infrared luminosities LIR
(and thus SFRobscured) and dust masses (subsequently MISM, assuming a given
gas-to-dust ratio). To demonstrate the advantage of a multi-wavelength
approach, we estimated the accuracy one would achieve while deriving LIR and
Mdust as a function of redshift for different available bands. After exploring
almost all possible band combinations, for the purposes of this work we
considered the following three cases: (i) galaxies only detected at 2000m,
(ii) galaxies only detected at 350m, and (iii) galaxies detected at (350 or
450) m and (750 or 850 or 1100) m and (1300 or 2000 or 3000) m. To infer these
accuracies, we assumed that the diversity of SEDs in the Universe is given by
the SED library of Draine & Li (2007), and we fitted mock observations
(assuming signal-to-noise ratios of 5) of these templates with a blackbody
function (Tdust in the range 10-60K). In Fig. 2 we show the mean and
dispersion of the log10(LIR-BB / LIR-True) and log10(Mdust-BB / Mdust-true).
This shows that 350m is a good LIR proxy from $z\sim$1-8 (i.e. probing the
rest-frame 40-120m range of the SED), but a poor proxy for the dust mass. 2000
m tells the opposite story, i.e., a good dust mass proxy (because of the
Rayleigh-Jeans tail) but a poor LIR proxy. Finally, one can do very well for
both LIR and the dust mass if data in more than two bands are available, i.e.
the third case we considered. This is quantified in Fig. 2: the range of
$T_{dust}$ explored is that of the Draine & Li (2007) SED library, so it does
contain SEDs with high temperature. For detection, we used a signal-to-noise
limit of 5, and the red lines in 2 show that do better than 0.1 dex (25%) at
$z>1$.

Figure 3: Exploring the parameter space of a 1000 deg2 survey using 1000 hours
of AtLAST observing time. Top panel: SFR vs. redshift survey limit. Second
panel: $dn/dz$. The third panel shows the fraction of (mock) sources detected
on 1, 2, 3, …, or all 8 bands. The bottom panel shows the expected AtLAST
accuracy in inferring $L_{FIR}$ and $M_{dust}$ as a function of redshift
(better than 25% and 50% at $z>2$, respectively), demonstrating why multi-band
detections are important.

Using the expected mapping speed of AtLAST (fitted with a multi-chroic camera
with a million pixels, which is what we expect for our first generation
camera), a 1000 deg2 survey (with 1000 hours observing time) results in a 3
sensitivity limit of 570 Jy at 350 m (at this limit, 82% of the Cosmic
Infrared Background at 350m will be resolved into individual sources) and 324
Jy at 450 m. At lower frequencies we hit the confusion limit (e.g. Blain et
al. 2002). Note that this implies that going for a much smaller field will
only benefit the 350 m and and 450 m bands, and thus low-redshift galaxies
science, where much work has already been done. With these limits we use the
model of Béthermin et al. (2017) to explore the parameter space probed by this
survey, which is shown in Fig. 3.

The first panel of Fig. 3 shows the classic star formation rate (SFR) versus
redshift survey limit using the models of Béthermin et al. (2017) and
classical scaling relations. AtLAST will be able to detect sources below SFR∗
up to z$\sim$5, where SFR∗ is the characteristic SFR defined as the value at
the ’knee’ of the classical Schechter function. We compare this to what
Herschel and ALMA can do: for ALMA we considered a hypothetical 1000 hours
survey in one ALMA band only (either 350 m, 450 m or 3mm), over 10 deg2
(requiring tens of thousands of pointings at 0.85-1.3mm), assuming that the
ALMA sensitivity will be twice as good as it is now (bandwidth increased by a
factor 4, taking the ALMA WSU upgrade into account: Carpenter et al. 2023) and
no overheads. The shaded region shows the range of star formation rates probed
by such a survey. ALMA, with its small field of view, is clearly not
efficient. The most optimal survey with ALMA would be at 850um, still barely
reaching SFR∗ up to $z\sim$4\. The second panel of Fig. 3 displays $dn/dz$ for
our 1000 deg2 survey, whereas the third one shows the fraction of sources with
detection in only 1 band, only 2 bands, etc., up to all 8 bands. Over the full
redshift range, about 70-80% of our galaxies will have detection in at least 3
bands, and therefore can be used to simultaneously solve for LIR and dust
mass, using photometric redshift estimates from AtLAST itself and elsewhere.

With a facility like AtLAST, we will, for the first time, have the far-
infrared SED (LIR, MDUST, TDUST) of all the relevant star-forming galaxies
(SFR larger than halve of SFR∗) from $z\sim$0 to $z\sim$5-6. This will allow
statistically-sound studies on the star forming and inter-stellar matter
content of galaxies even while dividing our sample in many redshift, mass,
metallicity, environment, and morphology subsamples. Such a survey will prove
extremely valuable to complement already planned large optical/near-infrared
surveys from which our photometric redshifts will be drawn (combined with the
multi-band AtLAST data). The bottom panel of Fig. 3 shows the average LIR and
MDUST accuracies of our survey. This quantifies why having detections in
several bands for most of our galaxies is important (70-80% of the galaxies
will have at least bands, as shown in the third panel). We will have an
accuracy better than $\sim$0.3 dex for both LIR and MDUST for over 78 million
galaxies, which will be unprecedented. This survey does need good 350/450m
conditions, so observing should be spread over four years at least, most
likely. Interestingly, this continuum survey will contain many Virgo/Coma-like
structures up to $z\sim$2, and group/poor clusters up to $z\sim$6\. We discuss
how to obtain a large, complete cluster sample in Section 2.4.

Figure 4: Exploring the parameter space of a 1 deg2 deep line survey using
3000 hours of AtLAST observing time. The top panel shows, for each line
considered, the minimum star formation rate (SFR) a galaxy must have for be
detected at a given redshift (see main text for more details). The second
panel displays $dn/dz$. The third panel shows the fraction of (mock) sources
with only CII detection, only a single CO detection, only multiple CO
detection (no CII, no FSL).

####  A deep “blind” spectroscopic survey

The high density of spectral features and large spectroscopic bandwidths of
optical spectrographs make the optical regime an excellent wavelength range to
determine the redshifts of large samples of galaxies. For example, one of the
most ambitious new redshift surveys will be done using Euclid, with a target
number of 1.5$\times$108 galaxy redshifts (Laureijs et al., 2011). However,
optical redshifts provide a biased view, missing most of the dust obscured
objects. This is particularly important for the most highly star-forming
objects which tend to be obscured by their large reservoirs of interstellar
dust. The redshifts of these dusty star-forming galaxies (DSFGs) have been
first attempted with optical spectroscopy (e.g. Chapman et al., 2005), but it
has since become clear that direct mm spectroscopy is a much more exact and
efficient method (e.g. Weiss et al., 2007, Reuter et al., 2020, Chen et al.,
2022, Cox et al., 2023).

Therefore, in addition to a very wide continuum survey, we also estimate what
a deep line survey with AtLAST can achieve. We consider a factor of three
increase in time (3000 hours), which is realistic as a line survey uses mostly
the low-frequency part of the spectrum: the 350/450m bands do not provide many
lines but for CII at $z\sim$1.5-2.0. The exercise here is to see what a 3000
hours spectrocopic survey with AtLAST delivers.

Again using the expected mapping speed of AtLAST, for a deep 1 deg2 line
survey (with 3000 hours observing time) in a 400km/s channel (R=750), we can
estimate the sensitivity limits for the various typical bands. Not listing all
available bands, we find (assuming no confusion) 3 sensitivity limits ranging
from 2330 Jy at 350 m, 210 Jy at 850 m, to 27 Jy at 3 mm. With these limits we
employ the model of Béthermin et al. (2017) to model the galaxy population and
predict their CO and fine structure line (FSL) peak flux densities, assuming a
line width of $\sim$400 km/s (i.e., matching our channel width). We do this
for the CO lines from Jup=1 to Jup=7, assuming L’CO(1-0)/LIR = 40 and a sub-mm
galaxy CO ladder as in Tab. 2 of Carilli & Walter (2013). We take LCII/LIR
$\sim 3\times 10^{-3}$ (not in the deficit part as our survey will be
dominated by SFR∗ galaxies). Other FSLs are CI610, CI370, NII205, OI146,
NII122, OIII88, OI63 and OIII51, for which we use an FSLline/LIR ratio as
found in the literature Graciá-Carpio et al. (2011), Bothwell et al. (2016),
Zhao et al. (2016), Carilli & Walter (2013), Schimek et al. (2024).

Figure 5: A 900 deg2 portion of the map of the gravitational mass distorting
the Cosmic Microwave Background obtained using the new Atacama Cosmology
Telescope Data Release 6, overlayed with the Cosmic Infrared Background (CIB,
sampled at a frequency of 545 GHz by Planck) tracing the distribution of dusty
galaxies, peaking at $z\sim 0.5-5$. The comparison shows a perfect match
between the CMB lensing map and the sub-mm tail of the CIB as seen by Planck,
where the over-densities of the latter (orange contours) correspond well with
the bright peaks of the CMB lensing map. The white dashed circle shows the
2-deg-diameter field of view of AtLAST, and the inset shows the AtLAST
resolution element computed at 545 GHz ($2.4^{\prime\prime}$, the diffraction-
limited beam full width half power). Figure adapted from Madhavacheril et al.
(2023).

For this survey setup we again plot the parameter space probed by this line
survey, now in Fig. 4. The top panel shows, for any given line considered, the
minimum SFR a galaxy must have to be detected at a given redshift (for CO, the
thin line is for J=1-0, and the thickest line for J=7-6; for FSL, the thin
line is for CI610, and the thickest line for OIII51; i.e. plot line thickness
increases with increasing energy). Such a survey will basically provide
multiple line detection for galaxies below SFR∗ up to $z\sim$7\. In
particular, SFR∗ galaxies at $4<z<7$ should all have one CO detection and one
[CII] detection, reminding ourselves that multiple line detection is crucial
for redshift determination. Of course, [CII] at high redshift detects sources
well below SFR∗. One can argue that using energetic considerations, such
single line detections could be used on their own to constrain the redshift of
these sources. Note that ALMA will be a factor $\sim$ 50 less sensitive for a
3000 hrs / 1 deg2 survey. The second panel of Fig. 4 shows $dn/dz$ for our 1
deg2 mock line survey. Finally, the bottom panel of Fig. 4 shows the fraction
of sources with only [CII] detection, only a single CO detection, and only
multiple CO detection (no [CII], no FSL). About 90% and 50% of our galaxies at
$z<5$ and $z>5$, respectively, will have multiple line detections, which is
excellent.

All this means that an AtLAST 1 deg2/3000 hrs line survey (a 400 km/s channel,
i.e. R=750) will basically give us multiple line detection for SFR∗ galaxies
up to $z\sim$7\. Combining this with multiple band continuum detections will
allow us to obtain a wealth of information for each of these SFR∗ $z<7$
galaxies: the redshift, gas content, cooling budget, star formation rate, dust
mass, and dust temperature. A survey going much wider than 1 deg2 in the same
amount of time will loose many of the CO lines, but will still detect [CII].

###  Constraining cosmological parameters via BAO and clustering

One of the most fundamental issues in modern cosmology is the significantly
different values of the Hubble constant measured from the CMB (the early
universe) and those based on late time observations, colloquially known as
’Hubble tension’ (see Di Valentino et al. 2021 for a review). Improving and
expanding the state-of-the-art measurement methods is one way to tackle the
Hubble tension. Clustering of galaxies measured in large samples with
spectroscopic redshifts provides a powerful means in this regard, and has been
shown to be one of the standard references since the first significant
measurements of the redshift space distortion (RSD; Peacock et al. 2001) and
the Baryon Acoustic Oscillation (BAO, sound waves from the embryonic Universe;
Cole et al. 2005). This effective approach has been realized and the
cosmological parameters, especially the growth rate, f8, and the Hubble
constant, have been measured to percent level precision up to $z\lesssim 1$
(Bautista et al., 2021, DES Collaboration et al., 2024), and the new surveys
that will be carried out by Dark Energy Spectroscopic Instrument and the
Subaru Prime Focus Spectrograph will push that limit to $z\sim 2.4$ (Takada et
al., 2014, DESI Collaboration et al., 2016).

Similar measurements beyond $z\sim 2.4$ may start to become challenging for
optical and near infrared observations, since most of the strong lines move
toward longer wavelengths that are hard to access from the ground, and as
galaxies become fainter at higher redshifts and so do their lines.
Measurements in the FIR, on the other hand, become more easily accessible
thanks to the shifting of the bright [CII] and CO lines to the more
transparent atmospheric windows. Indeed, detecting these far-infrared fine-
structure lines from galaxies at $z>2$ has become a regular practice for
extragalactic studies in the early cosmic time (e.g., Béthermin et al. 2020,
Bouwens et al. 2022)

Figure 6: Left: Data points with errors are monopole and quadrupole moments of
the correlation functions measured using dark matter halos in a simulated dark
matter only box, and the curves are the best-fit models including four
parameters, $f{}_{8}$, $b{}_{8}$, $H(z)$ and $D_{A}$. Both BAO and RSD are
considered in these fittings. Data points are shown within the separation
ranges on which the fittings are performed. Right: The results of the MCMC
analyses with one and two sigma confidence regions colour coded by different
darkness of blue. Here $b{}_{8}$ and $D_{A}$ are marginalized over.

A high-redshift galaxy spectroscopic survey with AtLAST would combine the
successful strategy from the optical/infrared surveys that measured galaxy
clustering in the last few decades (eg. Wilkinson et al. 2017, Cochrane et al.
2018, Amvrosiadis et al. 2019, Johnston et al. 2021, Vakili et al. 2023, van
Kampen et al. 2023, and many references therein) with the new possibility of
measuring strong emission lines from the FIR regime for large samples of
galaxies at $z\gtrsim 3$.

To test the feasibility and help design such a survey, as a first step, we
have conducted a simple forecast study using dark matter only cosmological
simulation at $z=3$. We use $N$-body simulation runs that are part of the DARK
QUEST project (Nishimichi et al., 2019). The simulation box has 2 Gpc/h on a
side with the Planck cosmology as the fiducial cosmological model. Halos are
identified using the ROCKSTAR algorithm (Behroozi et al., 2013). The halo mass
is defined by a sphere with a radius $R_{\rm 200m}$ within which the enclosed
average density is 200 times the mean matter density, as $M_{h}\equiv M_{\rm
200m}$. Motivated by recent clustering measurements of dusty star-forming
galaxies at $z>1$ (Lim et al., 2020, Stach et al., 2021), we apply a selection
of halos with masses of $10^{12.5}-10^{13.5}$ solar masses, which yields about
800k halos for the analyses.

The two-point correlation functions are measured using the selected simulated
halos, and covariance matrix is estimated with the standard bootstrap method
(Norberg et al., 2009). The models considered for the fitting of the
correlation functions are the fiducial -CDM, Baryonic Accustic Occasilations
(BAO; Eisenstein et al. 2005), linear redshift space distortion (RSD; Kaiser
1987), and the Alcock-Paczynski effect (Alcock & Paczynski, 1979). In this
model, we have four free parameters in total, bias ($b{}_{8}$), growth rate
($f{}_{8}$), Hubble parameter ($H(z)$) and angular diameter distance
($D_{A}$). To break the degeneracy between bias and the growth rate, we
perform fittings of the monopole and the quadrapole moments of the measured
correlation functions, over a pair separation range of 25-140 Mpc/$h$ and
40-140 Mpc/$h$, respectively.

The results are plotted in Figure 6, where the left panel shows the results of
the fittings of the correlation functions and the right panel shows the MCMC
results of the cosmological parameters, including the Hubble parameter and the
growth rate. The other two parameters, bias and angular diameter distance, are
marginalized over. In summary, the Hubble parameter can be measured at 0.7%
precision and the growth rate at 7.3%.

The above analyses demonstrate the potential constraining power of a baseline
design of a AtLAST spectroscopic survey; that is, a spectroscopic survey of
hundreds of thousands of sources with a footprint of about 1000 square
degrees, which can be achieved with a survey of a few thousand hours when
employing the bright [CII] line, as shown in the previous sections.

Figure 7: Line-intensity mapping: predicted 3D power spectrum of the [CII]
emission at redshift $z=3$. Individual curves correspond to different
analytical (Yue & Ferrara, 2019, Yang et al., 2021) and N-body + semi-
analytical models (Béthermin et al., 2022, Lagos et al., 2020). We also show
two realisations of the SHARK semianalytical model coupled with two different
prescriptions for [CII] emission (De Looze et al., 2014, Lagache et al.,
2018). The predictions vary by over 3 dex; the large FoV and sensitivity of
AtLAST will be critical for measuring LIM signal across cosmic time. Adapted
from Béthermin et al. (2022), J. Hilhorst (MSc thesis).

###  Line-intensity mapping (tomography)

A novel method for studying the large-scale structure of the Universe is the
line-intensity mapping (LIM) of the [CII] and CO emission lines. Specifically,
line-intensity mapping measures 2- and 3-dimensional power spectra from
spectral cubes, providing a statistical view of the large-scale structure
across cosmic time. LIM experiments at sub-mm wavelengths bridge the optical
surveys at $z\leq 1$ and the upcoming radio surveys of the 21-cm HI emission
line in the Epoch of Reionisation ($z\geq 6$). Namely, the optical surveys
become inefficient at higher redshifts as the bright optical lines move into
infrared wavelengths and the dust obscuration increases; conversely, the 21-cm
line signal peters out at $z\leq 6$ as the neutral hydrogen in the
intergalactic medium becomes fully ionised. The [CII] and CO lines remain
bright across this redshift range and do not suffer from dust obscuration,
making them ideal tracers of the large-scale structure.

####  Predictions for LIM signal

The predictions for LIM mapping signal (and the associated foregrounds) have
been explored theoretically using different approaches: from simple analytical
models (e.g., Yue & Ferrara 2019) to dark-matter only simulations combined
with semi-analytical models (e.g., Béthermin et al. 2022) and cosmological-
volume hydrodynamical simulations (e.g., Karoumpis et al. 2022). The
predictions for the resulting 2D and 3D power spectra vary by _several orders
of magnitude_ (Fig. 7), chiefly due to differences in the assumptions on
galaxy evolution and predictions of emission line intensity.

####  Current observations: lack of constraints

Several teams have conducted early LIM experiments on 10-metre class
telescopes. CONCERTO, a scanning Martin-Puplett interferometer with MKID
detectors covering 125 – 310 GHz frequency range, was installed on the APEX
telescope in 2021 (CONCERTO Collaboration et al., 2020, Monfardini et al.,
2022), mapping $\approx$1.4 deg2. TIME is 16-pixel grating spectrometer
observing the 200–300 GHz band, mounted on the 12-m ARO telescope (Crites et
al., 2014, Li et al., 2018). At lower redshifts, COMAP (Cleary et al., 2022,
Lamb et al., 2022) is a 19-pixel heterodyne spectrometer mapping the
CO(1–0)/(2–1) emission lines in the 26–34 GHz band. These experiments are
currently limited by the small collecting areas, low pixel count, and small
survey areas (few deg2); rather than detecting the LIM signal at high
redshift, they will provide upper bounds, potentially ruling out some of the
more “extreme” models. AtLAST will supersede these facilities by providing a
much larger collecting area, large focal plane, and superior site and dish
quality.

One of the key challenges in measuring the 2/3D power spectra of, e.g., [CII]
emission, is the need to remove “interlopers”: either CO and [CI] lines from
lower-redshift galaxies, or [OIII] emitters at higher redshifts. This can be
achieved by several techniques, such as masking (known) foreground galaxies
(e.g., Béthermin et al. 2022) or using the periodicity of CO emission lines to
separate the CO and [CII] power spectra (e.g., Yue & Ferrara 2019).

The power of CO LIM to constrain the cosmic expansion history $H(z)$, i.e the
Hubble expansion rate as a function of redshift, is illustrated in Fig. 8,
especially for $z>3$. This figure, taken from Silva et al. (2021), and based
on the work of Bernal et al. (2019), compares using only Supernovae (SN),
galaxy surveys and the Lyman- forest (red lines) to a combination of these
with a LIM experiment measuring CO(1-0) over a 1000 deg2 area. These
calculations were not specifically performed for AtLAST, but do illustrate
well the how much LIM with AtLAST can contribute to constraining the cosmic
expansion history.

Figure 8: Model-independent constraints on the shape of the cosmic expansion
history, $E(z)=H(z)/H_{0}$ (normalised to CDM), with (blue lines) and without
(red lines) CO(1-0) Line-Intensity Mapping over a 1000 deg2 patch. For
details, see Bernal et al. (2019) and Silva et al. (2021), from which this
figure was taken. It emphasizes the importance of LIM in constraining the
cosmic expansion history, especially at $z>3$.

###  Surveying cluster galaxies in the distant Universe

Galaxy clusters are the first large structures to form and eventually evolve
into the largest virialised objects in the Universe. They should therefore be
seen as the earliest fingerprint of galaxy formation and evolution (e.g. the
review of Kravtsov & Borgani 2012). Clusters grow hierarchically through the
merging and accretion of smaller units of galaxy halos, which are dominated by
(very) young galaxies displaying intense bursts of star-formation — the dusty
star-forming galaxy population (DSFGs; see for a review Casey et al. 2014).
These are rich in molecular gas but also heavily obscured by dust, which makes
them prime targets for far-infrared/submm facilities (Alberts & Noble, 2022).
These early cluster galaxies are most probably the progenitors of elliptical
galaxies (eg. Lutz et al. 2001, Ivison et al. 2013) which end up dominating
local galaxy clusters. Fig. 9 (based on work by (Dannerbauer et al., 2014) on
a $z=2.2$ proto-cluster) shows how violent galaxy clusters could be in the
distant Universe. In this section we motivate the need of a systematic study
of the early galaxy population in proto-clusters in order to make big leaps
forward in this emerging research field, which we argue is best done with a
large single dish sub-mm telescope at a high site. Please note that we focus
here on the cluster galaxy population: a companion AtLAST case study by Di
Mascolo et al. (in prep.) focuses on probing the ICM using the full SZ
spectrum.

Figure 9: This artist’s impression depicts the formation of a galaxy cluster
in the early Universe. The galaxies are vigorously forming new stars and
interacting with each other. They are observed as Far-Infrared/Sub-mm Galaxies
or Dusty Star-Forming Galaxies. Credit: ESO/M. Kornmesser. Courtesy from ESO
Press Release October 2014.

####  A systematic mapping survey of distant cluster galaxies

Presently, samples of galaxies in proto-clusters are small and heterogenous,
cannot sample the full extent of the cluster infall regions, and take a lot of
time to complete using current facilities. For this reason there are still
relatively few cold ISM measurements of cluster galaxies. A future systematic
(sub-)mm survey of high-redshift (proto-)galaxy clusters will resolve this
situation, and allow us to answer the following scientific questions in
detail: 1) How do galaxies and clusters co-evolve at early times? 2) How does
environment (especially in over-dense regions) affect star formation,
enrichment, outflows and feedback processes? and 3) What is the time evolution
of each of these processes? When and where do they peak?

Currently the number of known spectroscopically confirmed (proto-)clusters
beyond $z=2$ is still relatively low (see for a review Overzier 2016), where
the targets are often sparsely sampled. The heterogeneous datasets collected
so far suffer from strong selection biases and projection effects (e.g. Chen
et al., 2023), which prevent obtaining a complete picture of the build-up of
the cluster galaxy population over cosmic time. To remedy this, a large and
statistical sample is required. However, each of these clusters and their
infall region cover a linear extension of up to 30 Mpc (Muldrew et al., 2015,
Casey, 2016, Lovell et al., 2018), which corresponds to about 30′ at $z\sim
1-7$. Therefore, to study and understand the epoch of cluster formation, one
really needs to cover areas of up to one square degree, something current sub-
mm facilities cannot practically do with adequate sensitivity and survey
speed.

For example, ALMA allows for a survey speed of at most a few square arcminutes
per hour for the brightest CO lines (Popping et al., 2016) to yield detections
for a sufficient number of cluster galaxies with L$>$ L∗ at $z\approx 1$. The
survey speed for a given line depends on the sensitivity to that line and the
primary beam at its frequency. This determines the number of pointings
required for a given desired map size: obtaining an area of a square degree
with ALMA would take at least 1000 hours for the line with the highest survey
speed, CO(5-4). This line is hard to interpret physically, while CO(3-2) would
take around 6000 hours with ALMA, and CO(2-1) would even need three times that
(Popping et al., 2016). This renders one degree surveys for the lower
transition lines prohibitively expensive with ALMA. Another property of ALMA
is the relatively narrow bandwidth (just below 8 GHz) which makes spectral
scans slow as well.

The past decade has seen a rise in several hundred detection’s of the cold
molecular gas supply that fuel the star formation in the distant Universe,
albeit focusing mostly on isolated field galaxies (Carilli & Walter, 2013,
Tacconi et al., 2018). However, the number of published cold gas measurements
of galaxies located in galaxy clusters at $z>1$ is fairly low (of order a
hundred). Even though ALMA and ATCA allowed this number to increase
significantly (Coogan et al., 2018, Dannerbauer et al., 2017, Hayashi et al.,
2017, 2018, Noble et al., 2017, 2019, Rudnick et al., 2017, Stach et al.,
2017, Tadaki et al., 2019, Jin et al., 2021, Williams et al., 2022, Cramer et
al., 2023, Alberts & Noble, 2022), it remains low nonetheless. In order to
resolve these problems described above, we aim to produce a high-redshift
counterpart to local large cluster galaxy surveys (eg. Abell et al. 1989).
This will help us understand the contribution of protoclusters to the obscured
cosmic star formation rate density evolution (Chiang et al., 2017).

Figure 10: Example mock image at 2.4 mm of the CO(3-2) flux of galaxies in and
around a simulated $z=1.74$ proto-cluster, including homogeneous background
noise and field galaxies along the line-of-sight. This $0.5\times 0.5$ degree
image constitutes a single pointing with a single-dish 50 m sub-mm telescope

####  The way forward

In order to make a big leap forward in understanding the evolution and
formation of the largest structures and galaxies, a sub-mm observatory
optimized for surveys is needed, i.e. a highly multiplexed instrument on a
telescope with a single dish of at least 50 m. To visualize what such a
telescope can achieve, a mock CO(3-2) image of a simulated proto-cluster at
$z=1.74$ is shown in Fig. 10 (homogeneous noise is added), observed in a
single pointing at 2.4 mm where the angular resolution would be of order of 12
arcsec for such a telescope, and up to 6$\times$ better at higher frequencies.
This mock image is derived from a light-cone constructed out of a semi-
analytical galaxy formation model (van Kampen et al., 2005) in which a cluster
simulation (using the same model set-up) was inserted at $z=1.74$ (a few
hundred cluster galaxies were added in this way, of which around 50 are
sufficiently bright to be detectable at the depth of this particular mock
image).

We will target galaxies in already confirmed galaxy clusters (with known
spectroscopic redshifts) as well as candidate clusters. In addition, we expect
to discover new proto-clusters, especially dust obscured ones with large
molecular gas reservoirs contained in their individual members, in the large
surveys planned for AtLAST. At the time of conducting this survey with a large
single dish telescope, a sample of several to a few ten thousand
(proto-)galaxy clusters from $z=1-10$ will exist coming from future surveys
and missions such as LSST and Euclid, the latter one with a dedicated study to
discover galaxy clusters at high redshift (Laureijs et al., 2011). Presently
we already have a few thousand known candidate (proto-)galaxy clusters from
Planck (Planck Collaboration et al., 2015, 2016) and Hyper Suprime-Cam
(Higuchi et al., 2019, Toshikawa et al., 2018).

##  Technical Justification

To meet the observational requirements outlined here, we perceive the most
salient instrumentation requirements to be the ability to achieve high mapping
speeds over large areas, in multiple bands for continuum surveys, and with
very wide bandwidths for emission line surveys. The current state of the art
Transition Edge Sensor (TES) bolometers or Kinetic Inductance Detectors (KIDs)
present the likeliest technological path to achieve this, as both of these are
of high technical readiness level, have demonstrated background-limited
performance in the mm/submm, and can be read out in large numbers (tens-
hundreds of thousands as noted in Klaassen et al. 2020) through frequency
multiplexing, allowing the construction of large imaging arrays and integral
field units. In the following subsections we discuss the technical
requirements for the science cases covered by this white paper.

###  AtLAST as a sub-mm redshift machine

Because DSFGs are dust-obscured, their redshifts are best obtained in the
mm/sub-mm wavebands instead of the classical optical or infrared parts of the
spectrum. As shown by e.g. Bakx & Dannerbauer (2022), a key requirement for
such direct mm spectroscopy is a very wide frequency coverage, which is
difficult to obtain with heterodyne receivers. Several dedicated broad-
bandwidth (but low spectral resolution) instruments were specifically designed
for redshift searches on single-dish (sub-)mm telescopes, such as Z-Spec
(Naylor et al., 2003), Zspectrometer (Harris et al., 2007), ZEUS (Ferkinhoff
et al., 2010), the Redshift Search Receiver (Erickson et al., 2007), or
DESHIMA (Endo et al., 2019, Taniguchi et al., 2022). These instruments have
demonstrated the technical feasibility of innovative technology, but still
resulted in only a few dozen new redshifts due to the limited sensitivity of
the telescopes they were mounted on. The ALMA Wideband Sensitivity Upgrade
(WSU; Carpenter et al., 2023) will improve ALMA’s capabilities for redshift
determinations, especially in Band 2 (Mroczkowski et al., 2019). However, due
to ALMA’s very limited primary beam, redshift searches are done mostly on
individual, known targets, or within very small fields.

AtLAST promises to make a leap forward thanks to its unique combination of
sensitivity, broad spectral bandwidth, wide field of view and multiplex
spectroscopic capabilites. Based on the prototype instruments described above,
the MKID-based IFUs will allow to cover frequency ranges of hundreds of GHz.
This very broad bandwidth will be possible as the requirements on spectral
resolution are rather low: $\sim$0.5 GHz will be sufficient to avoid line
smearing as the targets will be galaxies with line widths of several hundreds
to $>$1000 km s-1. Rather than covering a single object line the prototype
instruments mentioned above, the AtLAST IFUs will eventually cover the full
focal plane of 2∘ diameter.

The very broad spectral bandwidth is crucial for redshift determinations: for
example instrument covering the atmospheric windows from 125 to 500 GHz would
allow to use the brighter but more widely spaced FSLs rather than the fainter
CO lines222furthermore, FSL would mostly circumvent the potential redshift
degeneracies that follow the linear spacing of CO lines. for redshift
determinations (see Fig. 7 of the companion case study on CGM science by Lee
et al.). An additional advantage of FSL over the CO ladder is that they cover
a wider range of physical conditions in the gas clouds, covering not only the
PDR’s but also HII regions (see also Lee et al.); this will allow to obtain a
more complete census of the sources in an unbiased wide-field redshift survey.
Finally, very broad-band IFU studies will not only cover the emission lines,
but will also allow to determine the slope of the continuum emission which is
brighter at shorter wavelengths, providing an additional constraint on the
redshifts.

There exists the possibility for redshift degeneracies in a redshift survey
targeting CO lines (Bakx & Dannerbauer, 2022). To assess this, a tool is
available (https://github.com/tjlcbakx/redshift-search-graphs) to graph the
ability of an instrument to derive the redshift of a galaxy in each of the
redshift bins using solely the observational bandwidth of a receiver. It can
be used to determine the fraction of sources in a redshift regime that will
have no lines, one line, or multiple robust lines. Doing this exercise for the
2-3 mm windows shows that that a wide band IFU covering these windows would be
highly efficient at $z>2$.

For the line intensity mapping method we need a broad spectral coverage to map
the [CII] power spectrum, whereas we require a spectral resolution $\Delta
v=3000km/s$ ($R=100$) to measure the 3D power spectrum. This can be achieved
either with a dedicated low-resolution spectrometer, or by binning higher-
resolution spectra.

###  Surveying proto-cluster galaxies

To significantly increase the number of spectroscopically confirmed
(proto-)clusters galaxies requires a fast survey sub-mm telescope of at least
50-m. Such a size guarantees the unambiguous identification of cluster
galaxies due to the relatively high angular resolution. To get spectroscopic
redshifts of several hundred to thousand member galaxies per cluster, a
multiplex instrument with up to several thousand elements per field of view is
indispensable. One option would be a heterodyne instrument with a wide field
of view and extremely large spectral bandwidth, however the costs would be
exorbitant, so this will not be feasible. Thus, we opt for for the MKID
bolometer technology which should provide integral field unit spectroscopic
capabilities.

To guarantee spectroscopic redshifts from $z=1-10$ and a complete study of the
most prominent lines emitted from the cold ISM such as multiple CO, the two
[CI], the [CII], H2O and HCN lines, the spectrometer should have an
unprecedented bandwidth from 70 to 700 GHz. E.g., the brightest expected line
emitted in the far-infrared is [CII] at 158m. An instrument with spectral
coverage from 180 to 345 GHz could thus follow the early stages of cluster
evolution from $z=4.5-10$, whereas extending to higher frequencies ($\sim$700
GHz) would even allow us to map the peak of the star-formation and black hole
activity of the Universe at $z=2$ (Madau & Dickinson, 2014) with the same
line. Furthermore, such a set-up guarantees the detection of several CO lines
and the so-called CO SLED (spectral line energy distribution, e.g. Dannerbauer
et al. 2009, Daddi et al. 2015) can be established. This enables us to
securely determine physical properties such as the gas density, excitation
temperature and even molecular gas mass. In addition, with both [CI] lines the
total cold gas mass can be measured as well (Papadopoulos et al., 2004,
Tomassetti et al., 2014). Getting a complete picture of the cold ISM supported
by a large sample size is indispensable to study in a statistical way if
environment plays a role by measuring parameters such as star-formation
efficiency, molecular gas fraction and excitation.

We need a fast enough survey telescope that not only allows the study of
confirmed galaxy clusters (with known spectroscopic redshifts) and candidate
clusters but in addition will yield an unbiased survey (negelecting the impact
of cosmic variance) of a significant area of the sky (to beat cosmic variance)
within a reasonable time span. Therefore, to conduct the survey and achieve
the science goals following technical requirements should be fulfilled:
\- a bolometer based on millimeter KID technology with many thousands of
elements per field of view, which has spectroscopic properties similar to
multi-object spectroscopy in the optical and near-infrared
\- a large bandwidth from 70 to 700 GHz to obtain spectroscopic redshifts,
\- a field-of-view of $\sim$1 square degree to cover the typical size of
(proto-)galaxy clusters,
\- a spectral resolution of $500-1000$ km/s to detect cluster galaxies and
determine their redshifts (preferably from two or more lines),
-a survey speed of at least 15 arcmin2 per minute to obtain a statistically significant sample of several thousand galaxy clusters.

##  Summary and conclusions

In this paper we outline several high-z science cases for AtLAST, a future
50-m submillimeter telescope in the Atacama dessert, focusing on the overall
high-z galaxy population as well on galaxies in proto-clusters. Two companion
AtLAST case studies focus on emission line probes of the CGM of galaxies (Lee
et al., in prep.) and on probing the ICM using the full SZ spectrum (Di
Mascolo et al., in prep.).

AtLAST will have high angular resolution, a large collecting area and large
focal plane, and therefore a high survey speed (see e.g. Klaassen et al.,
2020, Ramasawmy et al., 2022, Mroczkowski et al., 2023, 2024). This means
AtLAST can cover large areas of the sky for high-$z$ galaxy surveys. A single
pointing will be of order 200 time larger than the LMT, for example. Also, the
photometric confusion noise at 350 m will be four orders of magnitude lower
than that of existing 6-m sub-mm telescopes. These mayor improvements on
existing facilities, combined with an excellent instrument suite, allows for a
large leap forward in the active research field of early galaxy formation and
evolution, and notably the study of Dusty Star-Forming Galaxies (DSFGs).

One or more large, homogeneous surveys (continuum or spectral line) of DSFGs
will yield classical statistical properties such as the auto-correlation
function, the (photometric) redshift distribution, number counts, and so
forth. It will also yield a high-$z$ counterpart to existing large galaxy
samples at low and intermediate redshifts, and make mayor contributions to the
understanding of how the evolution of DSFGs varies as a function the
environment (voids, filaments, groups, clusters). Additionally, such surveys
provide a rich catalogue of sources to follow-up with ALMA, JWST, and ELT.

Using a large multi-chroic camera allows for a comprehensive multi-band
imaging survey, uniquely mapping large parts of the sky to specifically target
high-$z$ galaxies and map their distribution, where several of the bands can
be observed simultaneously, allowing for accurate spectral slope
determinations and photometric redshifts (especially in combination with
complementary data available at other wavebands), for example. A worked
example for a 1000 deg2 continuum survey in 1000 hours was presented, which
for AtLAST will have a 3 sensitivity limit of 570 Jy at 350 m (at this limit,
82% of the Cosmic Infrared Background at 350m will be resolved into individual
sources) and 324 Jy at 450 m. We showed that the multi-wavelength approach
significantly increases the accuracy one would achieve while deriving LIR and
Mdust as a function of redshift for different available bands, especially if
data for three or more bands is available.

In addition to a very wide continuum survey, we also considered a deep line
survey with AtLAST using mostly the low-frequency part of the spectrum, which
includes [CII] all the way to $z\sim$8\. For a deep 1 deg2 line survey in 3000
hours, in a 400km/s channel (R=750), we can go down to 3 sensitivity limits of
2330 Jy at 350 m and 27 Jy at 3 mm. The model of Béthermin et al. (2017) was
used predict peak flux densities for CO and various fine structure lines, and
found that about 90% and 50% of our galaxies at $z<5$ and $z>5$, respectively,
will have multiple line detections, which is crucial for redshift
determination. Combining this with multiple band continuum detections will
allow us to obtain a wealth of information for each of these galaxies besides
the redshift: gas content, cooling budget, star formation rate, dust mass, and
dust temperature. Therefore, such deep, wide-bandwidth spectral line surveys
with AtLAST will be crucial for mapping the population of “normal” star-
forming galaxies and their gas content across the cosmic history. Also note
that especially the [CII] line will be very promising for this purpose as it
is one the brightest FIR lines out to high-$z$.

Besides studying the overall galaxy population at high redshifts, we can also
use wide and deep surveys of these galaxies to extract cosmological
parameters, especially the growth rate, f8, and the Hubble constant, by
measuring galaxy-galaxy clustering (including their monopole and quadrupole
moments) and fit cosmological models to these data. At $z\sim 2.5$ this
becomes difficult in the traditional optical/infrared bands, but high-redshift
galaxy spectroscopic survey with AtLAST would move the successful strategy of
the optical/infrared surveys to the mm/sub-mm part of the spectrum and combine
this with the new possibility of measuring strong emission lines from the FIR
regime at $z\gtrsim 3$. Testing this using simulated dark matter halos at
$z=3$ showed that we can measure the Hubble parameter at 0.7% precision and
the growth rate at 7.3%.

Another strong AtLAST science case revolves around Line-intensity mapping
(LIM), which provides an alternative method for studying the large-scale
structure of the Universe across cosmic time: it measures 2- and 3-dimensional
power spectra from spectral cubes. At sub-mm wavelengths this bridges the
optical surveys at $z\leq 1$ and the upcoming radio surveys of the 21-cm HI
emission line in the Epoch of Reionisation ($z\geq 6$). The [CII] and/or CO
lines remain bright for $1<z<6$, making them ideal LIM tracers. Current effort
are limited by small surveys areas, which AtLAST can resolve as it has a much
larger collecting area, large focal plane, and superior site and dish quality.
We demonstrated the power of LIM using a CO line to constrain the cosmic
expansion history $H(z)$, i.e the Hubble expansion rate as a function of
redshift. This is significantly improved for $z>3$ when one adds CO LIM to
information from Supernovae (SN), galaxy surveys and the Lyman- forest.

Finally, we also presented the science case for mapping several thousand
galaxy (proto)clusters at $z=1-10$ with AtLAST, producing a high-redshift
counterpart to local large surveys of rich clusters like the well-studied
Abell catalogue. The main aims of such a large survey of distant clusters are
the formation and evolution of cluster galaxies over cosmic time and the
impact of environment on the formation and evolution (possibly environmental)
of these galaxies. To make a big leap forward in this emerging research field,
we would need a large-format, wide-band, direct-detection spectrometer (based
on MKID technology, for example), covering a wide field of $\sim$1 square
degree and a frequency coverage from 70 to 700 GHz (which could be split over
two instruments).

In concluding, we have shown that AtLAST is able to yield significant progress
is a range of research topics focusing on the distant Universe and cosmology,
notably the overall high-z galaxy population, the ones located in proto-
clusters, and the measurement of several cosmological parameters to help
constrain cosmological models. This is made possible because of the high
angular resolution that a 50-meter aperture brings, its wide spectral coverage
with moderately high spectral resolution, and an excellent sensitivity that
can be reached over large patches of the mm/sub-mm sky, which is
unprecedented.

## Competing interests

No competing interests were disclosed.

## Grant information

This project has received funding from the European Union’s Horizon 2020
research and innovation programme under grant agreement No. 951815 (AtLAST).

F.M.M. acknowledges the UCM María Zambrano programme of the Spanish Ministry
of Universities funded by the Next Generation European Union and is also
partly supported by grant RTI2018-096188-B-I00 funded by the Spanish Ministry
of Science and Innovation/State Agency of Research
MCIN/AEI/10.13039/501100011033.

M.L. acknowledges support from the European Union’s Horizon Europe research
and innovation programme under the Marie Skłodowska-Curie grant agreement No
101107795.

L.D.M. is supported by the ERC-StG “ClustersXCosmo” grant agreement 716762.
L.D.M. further acknowledges financial contribution from the agreement ASI-INAF
n.2017-14-H.0. This work has been supported by the French government, through
the UCAJ.E.D.I. Investments in the Future project managed by the National
Research Agency (ANR) with the reference number ANR-15-IDEX-01.

M. R. is supported by the NWO Veni project "Under the lens" (VI.Veni.202.225).

S.W. acknowledges support by the Research Council of Norway through the EMISSA
project (project number 286853) and the Centres of Excellence scheme, project
number 262622 (“Rosseland Centre for Solar Physics”).

H.D. acknowledges financial support from the Agencia Estatal de Investigación
del Ministerio de Ciencia e Innovación (AEI-MCINN) under grant (La evolución
de los cúmulos de galaxias desde el amanecer hasta el mediodía cósmico) with
reference (PID2019-105776GB-I00/DOI:10.13039/501100011033) and del Ministerio
de Ciencia, Innovación y Universidades (MCIU/AEI) under grant (Construcción de
cúmulos de galaxias en formación a través de la formación estelar ocurecida
por el polvo) and the European Regional Development Fund (ERDF) with reference
(PID2022-143243NB-I00/DOI:10.13039/501100011033).

## Acknowledgements

## References

  * Abell et al. (1989) Abell, G. O., Corwin,Harold G., J., & Olowin, R. P. 1989, ApJS, 70, 1
  * Alberts & Noble (2022) Alberts, S., & Noble, A. 2022, Universe, 8, 554
  * Alcock & Paczynski (1979) Alcock, C., & Paczynski, B. 1979, Nature, 281, 358
  * Algera et al. (2023) Algera, H. S. B., Inami, H., Oesch, P. A., et al. 2023, MNRAS, 518, 6142
  * Amvrosiadis et al. (2019) Amvrosiadis, A., Valiante, E., Gonzalez-Nuevo, J., et al. 2019, MNRAS, 483, 4649
  * Bakx & Dannerbauer (2022) Bakx, T. J. L. C., & Dannerbauer, H. 2022, MNRAS, 515, 678
  * Barrufet et al. (2023) Barrufet, L., Oesch, P. A., Bouwens, R., et al. 2023, MNRAS, 522, 3926
  * Bautista et al. (2021) Bautista, J. E., Paviot, R., Vargas Magaña, M., et al. 2021, MNRAS, 500, 736
  * Behroozi et al. (2013) Behroozi, P. S., Wechsler, R. H., & Wu, H.-Y. 2013, ApJ, 762, 109
  * Bernal et al. (2019) Bernal, J. L., Breysse, P. C., & Kovetz, E. D. 2019, Phys. Rev. Lett., 123, 251301
  * Béthermin et al. (2015) Béthermin, M., De Breuck, C., Sargent, M., & Daddi, E. 2015, A&A, 576, L9
  * Béthermin et al. (2017) Béthermin, M., Wu, H.-Y., Lagache, G., et al. 2017, A&A, 607, A89
  * Béthermin et al. (2020) Béthermin, M., Fudamoto, Y., Ginolfi, M., et al. 2020, A&A, 643, A2
  * Béthermin et al. (2022) Béthermin, M., Gkogkou, A., Van Cuyck, M., et al. 2022, A&A, 667, A156
  * Blain et al. (2002) Blain, A. W., Smail, I., Ivison, R. J., Kneib, J. P., & Frayer, D. T. 2002, Phys. Rep., 369, 111
  * Boogaard et al. (2023) Boogaard, L. A., Decarli, R., Walter, F., et al. 2023, ApJ, 945, 111
  * Bothwell et al. (2016) Bothwell, M. S., Maiolino, R., Peng, Y., et al. 2016, MNRAS, 455, 1156
  * Bouwens et al. (2022) Bouwens, R. J., Smit, R., Schouws, S., et al. 2022, ApJ, 931, 160
  * Carilli & Walter (2013) Carilli, C. L., & Walter, F. 2013, ARA&A, 51, 105
  * Carpenter et al. (2023) Carpenter, J., Brogan, C., Iono, D., & Mroczkowski, T. 2023, in Physics and Chemistry of Star Formation: The Dynamical ISM Across Time and Spatial Scales, 304
  * Carraro et al. (2020) Carraro, R., Rodighiero, G., Cassata, P., et al. 2020, A&A, 642, A65
  * Casey (2016) Casey, C. M. 2016, ApJ, 824, 36
  * Casey et al. (2014) Casey, C. M., Narayanan, D., & Cooray, A. 2014, Phys. Rep., 541, 45
  * Casey et al. (2021) Casey, C. M., Zavala, J. A., Manning, S. M., et al. 2021, ApJ, 923, 215
  * Chapman et al. (2005) Chapman, S. C., Blain, A. W., Smail, I., & Ivison, R. J. 2005, ApJ, 622, 772
  * Chen et al. (2022) Chen, C.-C., Liao, C.-L., Smail, I., et al. 2022, ApJ, 929, 159
  * Chen et al. (2023) Chen, J., Ivison, R. J., Zwaan, M. A., et al. 2023, A&A, 675, L10
  * Chiang et al. (2017) Chiang, Y.-K., Overzier, R. A., Gebhardt, K., & Henriques, B. 2017, ApJ, 844, L23
  * Cleary et al. (2022) Cleary, K. A., Borowska, J., Breysse, P. C., et al. 2022, ApJ, 933, 182
  * Cochrane et al. (2018) Cochrane, R. K., Best, P. N., Sobral, D., et al. 2018, MNRAS, 475, 3730
  * Cole et al. (2005) Cole, S., Percival, W. J., Peacock, J. A., et al. 2005, MNRAS, 362, 505
  * Combes (2018) Combes, F. 2018, A&A Rev., 26, 5
  * CONCERTO Collaboration et al. (2020) CONCERTO Collaboration, Ade, P., Aravena, M., et al. 2020, A&A, 642, A60
  * Coogan et al. (2018) Coogan, R. T., Daddi, E., Sargent, M. T., et al. 2018, MNRAS, 479, 703
  * Cox et al. (2023) Cox, P., Neri, R., Berta, S., et al. 2023, A&A, 678, A26
  * Cramer et al. (2023) Cramer, W. J., Noble, A. G., Massingill, K., et al. 2023, ApJ, 944, 213
  * Crites et al. (2014) Crites, A. T., Bock, J. J., Bradford, C. M., et al. 2014, in Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII, 91531W
  * Daddi et al. (2015) Daddi, E., Dannerbauer, H., Liu, D., et al. 2015, A&A, 577, A46
  * Dannerbauer et al. (2009) Dannerbauer, H., Daddi, E., Riechers, D. A., et al. 2009, ApJ, 698, L178
  * Dannerbauer et al. (2014) Dannerbauer, H., Kurk, J. D., De Breuck, C., et al. 2014, A&A, 570, A55
  * Dannerbauer et al. (2017) Dannerbauer, H., Lehnert, M. D., Emonts, B., et al. 2017, A&A, 608, A48
  * Dayal et al. (2022) Dayal, P., Ferrara, A., Sommovigo, L., et al. 2022, MNRAS, 512, 989
  * De Looze et al. (2014) De Looze, I., Cormier, D., Lebouteiller, V., et al. 2014, A&A, 568, A62
  * DES Collaboration et al. (2024) DES Collaboration, Abbott, T. M. C., Adamow, M., et al. 2024, arXiv e-prints, arXiv:2402.10696
  * DESI Collaboration et al. (2016) DESI Collaboration, Aghamousa, A., Aguilar, J., et al. 2016, arXiv e-prints, arXiv:1611.00036
  * Di Cesare et al. (2023) Di Cesare, C., Graziani, L., Schneider, R., et al. 2023, MNRAS, 519, 4632
  * Di Valentino et al. (2021) Di Valentino, E., Mena, O., Pan, S., et al. 2021, Classical and Quantum Gravity, 38, 153001
  * Dole et al. (2006) Dole, H., Lagache, G., Puget, J. L., et al. 2006, A&A, 451, 417
  * Draine & Li (2007) Draine, B. T., & Li, A. 2007, ApJ, 657, 810
  * Drew & Casey (2022) Drew, P. M., & Casey, C. M. 2022, ApJ, 930, 142
  * Dunlop et al. (2017) Dunlop, J. S., McLure, R. J., Biggs, A. D., et al. 2017, MNRAS, 466, 861
  * Eales et al. (2010) Eales, S., Dunne, L., Clements, D., et al. 2010, PASP, 122, 499
  * Eisenstein et al. (2005) Eisenstein, D. J., Zehavi, I., Hogg, D. W., et al. 2005, ApJ, 633, 560
  * Endo et al. (2019) Endo, A., Karatsu, K., Tamura, Y., et al. 2019, Nature Astronomy, 3, 989
  * Erickson et al. (2007) Erickson, N., Narayanan, G., Goeller, R., & Grosslein, R. 2007, in From Z-Machines to ALMA: (Sub)Millimeter Spectroscopy of Galaxies, 71
  * Everett et al. (2020) Everett, W. B., Zhang, L., Crawford, T. M., et al. 2020, ApJ, 900, 55
  * Faisst et al. (2020) Faisst, A. L., Schaerer, D., Lemaux, B. C., et al. 2020, ApJS, 247, 61
  * Ferkinhoff et al. (2010) Ferkinhoff, C., Nikola, T., Parshley, S. C., et al. 2010, in Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V, 77410Y
  * Fujimoto et al. (2023) Fujimoto, S., Kohno, K., Ouchi, M., et al. 2023, arXiv e-prints, arXiv:2303.01658
  * Geach et al. (2017) Geach, J. E., Dunlop, J. S., Halpern, M., et al. 2017, MNRAS, 465, 1789
  * Graciá-Carpio et al. (2011) Graciá-Carpio, J., Sturm, E., Hailey-Dunsheath, S., et al. 2011, ApJ, 728, L7
  * Gralla et al. (2020) Gralla, M. B., Marriage, T. A., Addison, G., et al. 2020, ApJ, 893, 104
  * Harris et al. (2007) Harris, A. I., Baker, A. J., Jewell, P. R., et al. 2007, in From Z-Machines to ALMA: (Sub)Millimeter Spectroscopy of Galaxies, 82
  * Hayashi et al. (2017) Hayashi, M., Kodama, T., Kohno, K., et al. 2017, ApJ, 841, L21
  * Hayashi et al. (2018) Hayashi, M., Tadaki, K.-i., Kodama, T., et al. 2018, ApJ, 856, 118
  * Higuchi et al. (2019) Higuchi, R., Ouchi, M., Ono, Y., et al. 2019, ApJ, 879, 28
  * Hirashita & Il’in (2022) Hirashita, H., & Il’in, V. B. 2022, MNRAS, 509, 5771
  * Inami et al. (2022) Inami, H., Algera, H. S. B., Schouws, S., et al. 2022, MNRAS, 515, 3126
  * Ivison et al. (2013) Ivison, R. J., Swinbank, A. M., Smail, I., et al. 2013, ApJ, 772, 137
  * Jin et al. (2021) Jin, S., Dannerbauer, H., Emonts, B., et al. 2021, A&A, 652, A11
  * Johnston et al. (2021) Johnston, H., Joachimi, B., Norberg, P., et al. 2021, A&A, 646, A147
  * Kaiser (1987) Kaiser, N. 1987, MNRAS, 227, 1
  * Karoumpis et al. (2022) Karoumpis, C., Magnelli, B., Romano-Díaz, E., Haslbauer, M., & Bertoldi, F. 2022, A&A, 659, A12
  * Klaassen et al. (2020) Klaassen, P. D., Mroczkowski, T. K., Cicone, C., et al. 2020, in Ground-based and Airborne Telescopes VIII, 114452F
  * Klitsch et al. (2019) Klitsch, A., Péroux, C., Zwaan, M. A., et al. 2019, MNRAS, 490, 1220
  * Kravtsov & Borgani (2012) Kravtsov, A. V., & Borgani, S. 2012, ARA&A, 50, 353
  * Lagache et al. (2018) Lagache, G., Cousin, M., & Chatzikos, M. 2018, A&A, 609, A130
  * Lagos et al. (2020) Lagos, C. d. P., da Cunha, E., Robotham, A. S. G., et al. 2020, MNRAS, 499, 1948
  * Lamb et al. (2022) Lamb, J. W., Cleary, K. A., Woody, D. P., et al. 2022, ApJ, 933, 183
  * Lammers et al. (2022) Lammers, C., Hill, R., Lim, S., et al. 2022, MNRAS, 514, 5004
  * Laureijs et al. (2011) Laureijs, R., Amiaux, J., Arduini, S., et al. 2011, arXiv e-prints, arXiv:1110.3193
  * Li et al. (2018) Li, C.-T., Bradford, C. M., Crites, A., et al. 2018, in Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy IX, 107083F
  * Lim et al. (2020) Lim, C.-F., Chen, C.-C., Smail, I., et al. 2020, ApJ, 895, 104
  * Lovell et al. (2018) Lovell, C. C., Thomas, P. A., & Wilkins, S. M. 2018, MNRAS, 474, 4612
  * Lutz et al. (2001) Lutz, D., Dunlop, J. S., Almaini, O., et al. 2001, A&A, 378, 70
  * Madau & Dickinson (2014) Madau, P., & Dickinson, M. 2014, ARA&A, 52, 415
  * Madhavacheril et al. (2023) Madhavacheril, M. S., Qu, F. J., Sherwin, B. D., et al. 2023, arXiv e-prints, arXiv:2304.05203
  * Monfardini et al. (2022) Monfardini, A., Beelen, A., Benoit, A., et al. 2022, Journal of Low Temperature Physics, 209, 751
  * Mountrichas & Shankar (2023) Mountrichas, G., & Shankar, F. 2023, MNRAS, 518, 2088
  * Mroczkowski et al. (2019) Mroczkowski, T., De Breuck, C., Kemper, C., et al. 2019, arXiv e-prints, arXiv:1905.09064
  * Mroczkowski et al. (2023) Mroczkowski, T., Cicone, C., Reichert, M., et al. 2023, in 2023 XXXVth General Assembly and Scientific Symposium of the International Union of Radio Science (URSI GASS), 1–4
  * Mroczkowski et al. (2024) Mroczkowski, T., Gallardo, P. A., Timpe, M., et al. 2024, arXiv e-prints, arXiv:2402.18645
  * Muldrew et al. (2015) Muldrew, S. I., Hatch, N. A., & Cooke, E. A. 2015, MNRAS, 452, 2528
  * Naylor et al. (2003) Naylor, B. J., Ade, P. A. R., Bock, J. J., et al. 2003, in Millimeter and Submillimeter Detectors for Astronomy, 239–248
  * Nishimichi et al. (2019) Nishimichi, T., Takada, M., Takahashi, R., et al. 2019, ApJ, 884, 29
  * Noble et al. (2017) Noble, A. G., McDonald, M., Muzzin, A., et al. 2017, ApJ, 842, L21
  * Noble et al. (2019) Noble, A. G., Muzzin, A., McDonald, M., et al. 2019, ApJ, 870, 56
  * Norberg et al. (2009) Norberg, P., Baugh, C. M., Gaztañaga, E., & Croton, D. J. 2009, MNRAS, 396, 19
  * Oliver et al. (2012) Oliver, S. J., Bock, J., Altieri, B., et al. 2012, MNRAS, 424, 1614
  * Overzier (2016) Overzier, R. A. 2016, A&A Rev., 24, 14
  * Papadopoulos et al. (2004) Papadopoulos, P. P., Thi, W. F., & Viti, S. 2004, MNRAS, 351, 147
  * Peacock et al. (2001) Peacock, J. A., Cole, S., Norberg, P., et al. 2001, Nature, 410, 169
  * Planck Collaboration et al. (2015) Planck Collaboration, Aghanim, N., Altieri, B., et al. 2015, A&A, 582, A30
  * Planck Collaboration et al. (2016) Planck Collaboration, Ade, P. A. R., Aghanim, N., et al. 2016, A&A, 596, A100
  * Popping et al. (2016) Popping, G., van Kampen, E., Decarli, R., et al. 2016, MNRAS, 461, 93
  * Ramasawmy et al. (2022) Ramasawmy, J., Klaassen, P. D., Cicone, C., et al. 2022, in Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI, 1219007
  * Reuter et al. (2020) Reuter, C., Vieira, J. D., Spilker, J. S., et al. 2020, ApJ, 902, 78
  * Rudnick et al. (2017) Rudnick, G., Hodge, J., Walter, F., et al. 2017, ApJ, 849, 27
  * Salim & Narayanan (2020) Salim, S., & Narayanan, D. 2020, ARA&A, 58, 529
  * Schimek et al. (2024) Schimek, A., Decataldo, D., Shen, S., et al. 2024, A&A, 682, A98
  * Shirley et al. (2021) Shirley, R., Duncan, K., Campos Varillas, M. C., et al. 2021, MNRAS, 507, 129
  * Silva et al. (2021) Silva, M. B., Kovetz, E. D., Keating, G. K., et al. 2021, Experimental Astronomy, 51, 1593
  * Sommovigo et al. (2022) Sommovigo, L., Ferrara, A., Pallottini, A., et al. 2022, MNRAS, 513, 3122
  * Stach et al. (2017) Stach, S. M., Swinbank, A. M., Smail, I., et al. 2017, ApJ, 849, 154
  * Stach et al. (2021) Stach, S. M., Smail, I., Amvrosiadis, A., et al. 2021, MNRAS, 504, 172
  * Tacconi et al. (2020) Tacconi, L. J., Genzel, R., & Sternberg, A. 2020, ARA&A, 58, 157
  * Tacconi et al. (2018) Tacconi, L. J., Genzel, R., Saintonge, A., et al. 2018, ApJ, 853, 179
  * Tadaki et al. (2019) Tadaki, K.-i., Kodama, T., Hayashi, M., et al. 2019, PASJ, 71, 40
  * Takada et al. (2014) Takada, M., Ellis, R. S., Chiba, M., et al. 2014, PASJ, 66, R1
  * Taniguchi et al. (2022) Taniguchi, A., Bakx, T. J. L. C., Baselmans, J. J. A., et al. 2022, Journal of Low Temperature Physics, 209, 278
  * Tomassetti et al. (2014) Tomassetti, M., Porciani, C., Romano-Diaz, E., Ludlow, A. D., & Papadopoulos, P. P. 2014, MNRAS, 445, L124
  * Toshikawa et al. (2018) Toshikawa, J., Uchiyama, H., Kashikawa, N., et al. 2018, PASJ, 70, S12
  * Vakili et al. (2023) Vakili, M., Hoekstra, H., Bilicki, M., et al. 2023, A&A, 675, A202
  * van Kampen et al. (2005) van Kampen, E., Percival, W. J., Crawford, M., et al. 2005, MNRAS, 359, 469
  * van Kampen et al. (2023) van Kampen, E., Lacy, M., Farrah, D., et al. 2023, MNRAS, 523, 251
  * Walter et al. (2016) Walter, F., Decarli, R., Aravena, M., et al. 2016, ApJ, 833, 67
  * Weiss et al. (2007) Weiss, A., Downes, D., Walter, F., & Henkel, C. 2007, in From Z-Machines to ALMA: (Sub)Millimeter Spectroscopy of Galaxies, 25
  * Weiß et al. (2009) Weiß, A., Kovács, A., Coppin, K., et al. 2009, ApJ, 707, 1201
  * Wilkinson et al. (2017) Wilkinson, A., Almaini, O., Chen, C.-C., et al. 2017, MNRAS, 464, 1380
  * Williams et al. (2022) Williams, C. C., Alberts, S., Spilker, J. S., et al. 2022, ApJ, 929, 35
  * Wolfire et al. (2022) Wolfire, M. G., Vallini, L., & Chevance, M. 2022, ARA&A, 60, 247
  * Yang et al. (2021) Yang, S., Somerville, R. S., Pullen, A. R., et al. 2021, ApJ, 911, 132
  * Yue & Ferrara (2019) Yue, B., & Ferrara, A. 2019, MNRAS, 490, 1928
  * Zanella et al. (2018) Zanella, A., Daddi, E., Magdis, G., et al. 2018, MNRAS, 481, 1976
  * Zhao et al. (2016) Zhao, Y., Lu, N., Xu, C. K., et al. 2016, ApJ, 819, 69


Electronic and Electrical Engineering, University College London, UK. His
current research centers around 5G and beyond mobile communications. He is a
co-recipient of the 2013 IEEE Signal Processing Letters Best Paper Award and
the 2000 IEEE VTS Japan Chapter Award at the IEEE Vehicular Technology
Conference in Japan in 2000, and a few other international best paper awards.
He is Fellow of IEEE and IET and is also on the editorial board of several
international journals. He is the Editor-in-Chief for IEEE Wireless
Communications Letters since 2020.
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|  Chan-Byoung Chae (Fellow, IEEE) received the Ph.D. degree in electrical and
computer engineering from The University of Texas at Austin (UT), USA in 2008.
Prior to joining UT, he was a Research Engineer at the Telecommunications
Research and Development Center, Samsung Electronics, Suwon, South Korea, from
2001 to 2005. He is currently an Underwood Distinguished Professor with the
School of Integrated Technology, Yonsei University, South Korea. Before
joining Yonsei University, he was with Bell Labs, Alcatel-Lucent, Murray Hill,
NJ, USA, from 2009 to 2011, as a Member of Technical Staff, and Harvard
University, Cambridge, MA, USA, from 2008 to 2009, as a Post-Doctoral Research
Fellow. Dr. Chae was a recipient/co-recipient of the KICS Haedong Scholar
Award in 2023, the CES Innovation Award in 2023, the IEEE ICC Best Demo Award
in 2022, the IEEE WCNC Best Demo Award in 2020, the Best Young Engineer Award
from the National Academy of Engineering of Korea (NAEK) in 2019, the IEEE
DySPAN Best Demo Award in 2018, the IEEE/KICS Journal of Communications and
Networks Best Paper Award in 2018, the IEEE INFOCOM Best Demo Award in 2015,
the IEIE/IEEE Joint Award for Young IT Engineer of the Year in 2014, the KICS
Haedong Young Scholar Award in 2013, the IEEE Signal Processing Magazine Best
Paper Award in 2013, the IEEE ComSoc AP Outstanding Young Researcher Award in
2012, and the IEEE VTS Dan. E. Noble Fellowship Award in 2008. Dr. Chae has
held several editorial positions, including Editor-in-Chief of the IEEE
Transactions on Molecular, Biological, and Multi-Scale Communications, Senior
Editor of the IEEE Wireless Communications Letters, and Editor for the IEEE
Communications Magazine, IEEE Transactions on Wireless Communications, and
IEEE Wireless Communications Letters. He is an IEEE ComSoc Distinguished
Lecturer from 2020 to 2023. He is an IEEE Fellow and NAEK Fellow.
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