CO emission in distant galaxies on and above the main sequence
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We present the detection of multiple carbon monoxide CO line transitions
with ALMA in a few tens of infrared-selected galaxies on and above the
main sequence at z = 1.1-1.7. We reliably detected the emission of CO (5
- 4), CO (2 - 1), and CO (7 - 6)+[C I](3P2 -
3P1) in 50, 33, and 13 galaxies, respectively, and
we complemented this information with available CO (4 - 3) and [C
I](3P1 - 3P0) fluxes for
part of the sample, and by modeling of the optical-to-millimeter
spectral energy distribution. We retrieve a quasi-linear relation
between LIR and CO (5 - 4) or CO (7 - 6) for main-sequence
galaxies and starbursts, corroborating the hypothesis that these
transitions can be used as star formation rate (SFR) tracers. We find
the CO excitation to steadily increase as a function of the star
formation efficiency, the mean intensity of the radiation field warming
the dust (⟨U⟩), the surface density of SFR
(ΣSFR), and, less distinctly, with the distance from
the main sequence (ΔMS). This adds to the tentative evidence for
higher excitation of the CO+[C I] spectral line energy distribution
(SLED) of starburst galaxies relative to that for main-sequence objects,
where the dust opacities play a minor role in shaping the high-J CO
transitions in our sample. However, the distinction between the average
SLED of upper main-sequence and starburst galaxies is blurred, driven by
a wide variety of intrinsic shapes. Large velocity gradient radiative
transfer modeling demonstrates the existence of a highly excited
component that elevates the CO SLED of high-redshift main-sequence and
starbursting galaxies above the typical values observed in the disk of
the Milky Way. This excited component is dense and it encloses ∼50%
of the total molecular gas mass in main-sequence objects. We interpret
the observed trends involving the CO excitation as to be mainly
determined by a combination of large SFRs and compact sizes, as a large
ΣSFR is naturally connected with enhanced dense
molecular gas fractions and higher dust and gas temperatures, due to
increasing ultraviolet radiation fields, cosmic ray rates, as well as
dust and gas coupling. We release the full data compilation and the
ancillary information to the community.
The data compilation described in Table D.1 is only available at the CDS
via anonymous ftp to http://cdsarc.u-strasbg.fr
(ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/641/A155
Original language | English |
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Article number | A155 |
Journal | Astronomy & Astrophysics |
Volume | 641 |
Number of pages | 21 |
ISSN | 0004-6361 |
DOIs | |
Publication status | Published - 1 Sep 2020 |
- galaxies: evolution, galaxies: ISM, galaxies: starburst, galaxies: high-redshift
Research areas
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