The ALMA Spectroscopic Survey in the HUDF: CO Excitation and Atomic Carbon in Star-Forming Galaxies at $z=1-3$. (arXiv:2009.04348v1 [astro-ph.GA])

The ALMA Spectroscopic Survey in the HUDF: CO Excitation and Atomic Carbon in Star-Forming Galaxies at $z=1-3$. (arXiv:2009.04348v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Boogaard_L/0/1/0/all/0/1">Leindert A. Boogaard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Werf_P/0/1/0/all/0/1">Paul van der Werf</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wei%5Css_A/0/1/0/all/0/1">Axel Weiß</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Popping_G/0/1/0/all/0/1">Gergö Popping</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Decarli_R/0/1/0/all/0/1">Roberto Decarli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Walter_F/0/1/0/all/0/1">Fabian Walter</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Aravena_M/0/1/0/all/0/1">Manuel Aravena</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bouwens_R/0/1/0/all/0/1">Rychard Bouwens</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Riechers_D/0/1/0/all/0/1">Dominik Riechers</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gonzalez_Lopez_J/0/1/0/all/0/1">Jorge González-López</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carilli_I/0/1/0/all/0/1">Ian Smail Chris Carilli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kaasinen_M/0/1/0/all/0/1">Melanie Kaasinen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Daddi_E/0/1/0/all/0/1">Emanuele Daddi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cox_P/0/1/0/all/0/1">Pierre Cox</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Diaz_Santos_T/0/1/0/all/0/1">Tanio Díaz-Santos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Inami_H/0/1/0/all/0/1">Hanae Inami</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cortes_P/0/1/0/all/0/1">Paulo C. Cortes</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wagg_J/0/1/0/all/0/1">Jeff Wagg</a>

We investigate the CO excitation and interstellar medium (ISM) conditions in
a cold gas mass-selected sample of 22 star-forming galaxies at z=0.46-3.60,
observed as part of the ALMA Spectroscopic Survey in the Hubble Ultra Deep
Field (ASPECS). Combined with VLA follow-up observations, we detect a total of
34 CO J -> J-1 transitions with J=1 up to 8 (and an additional 21 upper limits,
up to J=10) and six [C I] 3P_1 -> 3P_0 and 3P_2 -> 3P_1 transitions (and 12
upper limits). The CO(2-1) and CO(3-2)-selected galaxies, at z=1.2 and 2.5,
respectively, exhibit a range in excitation in their mid-J=4,5 and high-J=7,8
lines, on average lower than (L_IR-brighter) BzK-color- and
submillimeter-selected galaxies at similar redshifts. The former implies that a
warm ISM component is not necessarily prevalent in gas mass-selected galaxies
at z=1.2. We use stacking and Large Velocity Gradient models to measure and
predict the average CO ladders at z<2 and z>=2, finding r_21=0.75+/-0.11 and
r_31=0.77+/-0.14, respectively. From the models, we infer that the galaxies at
z>=2 have intrinsically higher excitation than those at z<2. This fits a
picture in which the global excitation is driven by an increase in the star
formation rate surface density of galaxies with redshift. We derive a neutral
atomic carbon abundance of (1.9+/-0.4) x 10^{-5}, comparable to the Milky Way
and main-sequence galaxies at similar redshifts, and fairly high densities (>=
10^4 cm^3), consistent with the low-J CO excitation. Our results imply a
decrease in the cosmic molecular gas mass density at z>=2 compared to previous
ASPECS measurements.

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