Automated Mining of the ALMA Archive in the COSMOS Field (A3COSMOS): II.
  Cold Molecular Gas Evolution out to Redshift 6
  We present new measurements of the cosmic cold molecular gas evolution out to
redshift 6 based on systematic mining of the ALMA public archive in the COSMOS
deep field (A3COSMOS). Our A3COSMOS dataset contains ~700 galaxies (0.3 < z <
6) with high-confidence ALMA detections in the (sub-)millimeter continuum and
multi-wavelength spectral energy distributions (SEDs). Multiple gas mass
calibration methods are compared and biases in band conversions (from observed
ALMA wavelength to rest-frame Rayleigh-Jeans(RJ)-tail continuum) have been
tested. Combining our A3COSMOS sample with ~1,000 CO-observed galaxies at 0 < z
< 4 (75% at z < 0.1), we parameterize galaxies' molecular gas depletion time
and molecular gas to stellar mass ratio (gas fraction) each as a function of
the stellar mass, offset from the star-forming main sequence (Delta MS) and
cosmic age (or redshift). Our proposed functional form provides a statistically
better fit to current data (than functional forms in the literature), and
implies a "downsizing" effect (i.e., more-massive galaxies evolve earlier than
less-massive ones) and "mass-quenching" (gas consumption slows down with cosmic
time for massive galaxies but speeds up for low-mass ones). Adopting galaxy
stellar mass functions and applying our function for gas mass calculation, we
for the first time infer the cosmic cold molecular gas density evolution out to
redshift 6 and find agreement with CO blind surveys as well as semi-analytic
modeling. These together provide a coherent picture of cold molecular gas, SFR
and stellar mass evolution in galaxies across cosmic time.
