Characterizing the physical conditions of cold/warm gas in submillimeter brightgalaxies at z > 3Ken-ichi Tadaki1
1National Astoronomical Observatory of Japan, 2–21–1 Osawa,Mitaka-shi, Tokyo 181–8588
tadaki.ken@nao.ac.jp

Abstract
Recent ALMA observations have revealed CO and [CI] spectral line energy distributions (SLED) including
mig–J/high–J CO lines and [CI](1-0)/[CI](2-1) lines in submillimeter bright galaxies at z > 3. It is importantto understand the physical conditions of cold and warm gas in these starburst galaxies, where the core of giant
ellipticals is being formed. Combining ngVLA observations of CO(1–0) and CO(2–1) emission lines with ALMA
ones, it possible to separately characterize the physical properties of cold and warm molecular gas and then measure
the gas mass without assuming a CO-to-H2 conversion factor and local thermodynamical equilibrium excitation.
Furthermore, the ngVLA-ALMA synergetic observations will reveal the spatial distributions of cold and warm gas,
separately.
Key words: galaxies: high-redshift — galaxies: starburst — galaxies: ISM
1. IntroductionRecent deep observations have spectroscopically identified
submillimeter bright galaxies (SMGs) at z = 3 − 6 (Riecherset al. 2013; Vieira et al. 2013; Yun et al. 2015; Casey et al.
2017; Casey et al. 2019). They are intensively forming stars at
more than 1000 times higher rate than the Milky Way and havelarge gas reservoirs to fuel star formation. They are also themost massive galaxies with a stellar mass of log(M⋆ /M⊙ ) ∼11 in the early Universe (Michałowski et al. 2014). A large
fraction of stars is formed in the central 1 kpc region (Simpsonet al. 2015; Ikarashi et al. 2015; Hodge et al. 2016), that iscomparable in size to massive, quiescent galaxies at the peakof cosmic star formation history (van Dokkum et al. 2015) anda core of giant elliptical galaxies in the present-day Universe.
The compact core must play a primary role in the bulge formation in the most massive galaxies in the early Universe while
the physical properties inside the compact starburst region arepoorly understood. It is important to understand how SMGs
build up a compact core through studies of the physical properties of molecular gas.
COSMOS-AzTEC-1 is a well studied SMG with a wide array of ancillary data, and can serve as a template for starbursts at high-redshift. This is an unlensed SMG at zspec =4.342 (Yun et al. 2015) and intrinsically has a large stel10
lar mass of M⋆ = 9.9+0.4
M⊙ (Tadaki et al. 2018).
−2.6 × 10
Five emission lines (CO(4–3), CO(7–6), CO(12–11), [C I](1–0) and [C I](2–1)) have been detected by ALMA observations(Tadaki et al. 2018; Tadaki et al. 2019). High-J CO lines
can be used for tracing warm gas while [C I] lines can beused for tracing cold gas (Weiß et al. 2005; Brisbin et al.
2019; Valentino et al. 2020). The observed CO and [C I] SLED
can not be explained by single component models, indicatingthat COSMOS-AzTEC-1 has at least two gas components (coldand warm gas). We also find that a high carbon abundance of[C I]/H2 > 1 × 10−4 is required to reproduce the CO and [C I]
SLED even with two component models (Figure 1). The typical value in normal star-forming galaxies is [C I]/H2 = 3×10−5(Papadopoulos & Greve 2004; Weiß et al. 2003; Weiß et al.
2005) and the value in the central region of the local starburst
galaxy M82 is [C I]/H2 = 5 × 10−5 . Such an elevated carbon
abundance has been pointed out in Bothwell et al. 2017.
2. Spatially-unresolved observations of CO(1–0) andCO(2–1) line emission
A problem is that the uncertainty on carbon abundance significantly affects the estimate of the gas column density andgas mass. In the case of COSMOS-AzTEC-1 (Figure 1), theestimated gas column density is NCO = 8 × 1018 cm−2 for[C I]/H2 = 5 × 10−5 (M82-like) and NCO = 6 × 1017 cm−2for [C I]/H2 = 2 × 10−4 . Thus, observations of CO(1–0) andCO(2–1) emission allow us to characterize the physical conditions of cold molecular gas as well as to constrain the carbonabundance. We require the sensitivity of 1σ=96 µJy beam−1
per 50 km s−1 at 21 GHz (for CO(1–0)) and 1σ=180 µJy
beam−1 per 20 km s−1 at 42 GHz (for CO(2–1)) (Figure 1).
Using ngVLA, we can reach these depths with about 20 minutes on source time and make a systematic survey of CO(1–0)
and CO(2–1) line emission for a large sample of bright SMGs
at z > 3. It is in principle possible to calculate the molecular
gas mass from only two [C I] lines. However, this approachrequires the assumptions that both [C I] lines are optically thin
and are in local thermodynamical equilibrium (LTE), which are
not verified by observations of high-redshift starburst galaxies.
The proposed study of CO and [C I] SLED through non-LTE
modeling will enable us to derive the optical depth and the excitation of these lines. Therefore, we will be able to measure
the gas mass without assuming a CO-to-H2 conversion factorand LTE excitation.

2[Vol. ,Line luminosity [L⊙ ]XCI =5e-5 (M82-like)

XCI =1e-4

XCI =2e-4

109

109

109

108

108

108

107

107

CO (predicted)
CO (ALMA)[CI] (ALMA)

107
2

4

6 8 10 12 14Upper J level24

6 8 10 12 14Upper J level24

6 8 10 12 14Upper J level

Fig. 1. CO and [C I] SLEDs of COSMOS-AzTEC-1 at z = 4.3. Red circles and magenta pentagons show the galaxy-integrated luminosities of CO(4–3),
CO(7–6), CO(12–11), [C I](1–0) and [C I](2–1) lines from the ALMA observations (Tadaki et al. 2018; Tadaki et al. 2019). From left to right, we show
the best-fit models (blue dashed lines for cold component, red dashed lines for warm one and black solid lines for the total) of the RADEX (van der Taket al. 2007) non-LTE modeling results in three cases of carbon abundance. Green circles show the predicted luminosities of CO(1–0) and CO(2–1) lines
in three cases of [C I] abundance.

3. Spatially-resolved observations of CO(1–0) and CO(2–1) line emission
A next step is to spatially resolve the CO(1–0) and CO(2–1) line emission, tracing cold gas. Hodge et al. 2012 haverevealed the spatial distribution of CO(2–1) line emission inGN20, a bright SMG at z = 4.2 (Hodge et al. 2012), and thisis the only success so far. As the difficulty mainly comes froma low luminosity and an extended structure, it is challenging tospatially resolve the low-J CO emission at 1 kpc-(0.′′ 1–0.′′ 2)
resolution with the current JVLA sensitivity. Figure 2 shows
the results from simulations of ngVLA observations with 8hours integration. The input model has the same CO excitation as in GN20 (Carilli et al. 2010; Carilli et al. 2011) andan exponential disk with 0.′′ 4 (2.7 kpc), which is a factor of
2–3 larger than in the CO(4–3) and CO(7–6) emission. The
ngVLA-ALMA synergetic observations will enable us to constrain the spatially-resolved CO SLEDs to separately map thedistributions of cold and warm gas component as well as tocharacterize the gas density and the gas kinetic temperature ofeach component.
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No. ]

3y-offset (arcsec)+1.5+1.0+1.0+0.5+0.500-0.5-0.5-1.0+1.50ngVLA/CO(2-1)

-1.0

2 kpc

-1.5
+1.5 +1.0 +0.5

y-offset (arcsec)+1.5

ngVLA/CO(1-0)-0.5 -1.0 -1.5

-1.5
+1.5 +1.0 +0.5
+1.5

ALMA/CO(4-3)

0-0.5 -1.0 -1.5
+1.5

ALMA/CO(7-6)

+1.0

+1.0

+1.0

+0.5

+0.5

+0.5

000-0.5

-0.5

-0.5

-1.0-1.0

-1.0

-1.5
+1.5 +1.0 +0.5

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x-offset (arcsec)-1.5+1.5 +1.0 +0.5

0-0.5 -1.0 -1.5x-offset (arcsec)

ALMA/CO(12-11)-1.5+1.5 +1.0 +0.5

0-0.5 -1.0 -1.5

x-offset (arcsec)

Fig. 2. The 0.′′ 2 (1.3 kpc)-resolution velocity-integrated flux maps of CO(1–0), CO(2–1), CO(4–3), CO(7–6), and CO(12–11) line emission are displayed.
The CO(1–0) and CO(2–1) maps are created by simulations with CASA/simobserve task while the CO(4–3), CO(7–6) and CO(12–11) maps are real
data from ALMA observations of COSMOS-AzTEC-1 at z = 4.3 (Tadaki et al. 2018; Tadaki et al. 2019). Contours are plotted every 2σ from 3σ.