We present the first results of an ALMA survey of the lower fine structure
line of atomic carbon [C I](^3P_1\,-\,^{3}P_0) in far infrared-selected
galaxies on the main sequence at z∼1.2 in the COSMOS field. We compare our
sample with a comprehensive compilation of data available in the literature for
local and high-redshift starbursting systems and quasars. We show that the [C
I](3P1​→3P0​) luminosity correlates on global scales with the
infrared luminosity LIR​ similarly to low-J CO transitions. We report
a systematic variation of L'_{\rm [C\,I]^3P_1\,-\, ^3P_0}/LIR​ as a
function of the galaxy type, with the ratio being larger for main-sequence
galaxies than for starbursts and sub-millimeter galaxies at fixed LIR​.
The L'_{\rm [C\,I]^3P_1\,-\, ^3P_0}/LCO(2−1)′​ and M[CI]​/Mdust​ mass ratios are similar for main-sequence galaxies and for
local and high-redshift starbursts within a 0.2 dex intrinsic scatter,
suggesting that [C I] is a good tracer of molecular gas mass as CO and dust. We
derive a fraction of f[CI]​=M[CI]​/MC​∼3−13%
of the total carbon mass in the atomic neutral phase. Moreover, we estimate the
neutral atomic carbon abundance, the fundamental ingredient to calibrate [C I]
as a gas tracer, by comparing L'_{\rm [C\,I]^3P_1\,-\, ^3P_0} and available
gas masses from CO lines and dust emission. We find lower [C I] abundances in
main-sequence galaxies than in starbursting systems and sub-millimeter
galaxies, as a consequence of the canonical αCO​ and gas-to-dust
conversion factors. This argues against the application to different galaxy
populations of a universal standard [C I] abundance derived from highly biased
samples.Comment: 14 pages + Appendix. Accepted for publication in ApJ. All the data
tables in Appendix will be also released in electronic forma