The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Evolution of the Molecular Gas in CO-selected Galaxies
2019
We analyze the interstellar medium properties of a sample of 16 bright CO line emitting galaxies identified in the
ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) Large Program. This CO−selected galaxy
sample is complemented by two additional CO line emitters in the UDF that are identified based on their MultiUnit Spectroscopic Explorer (MUSE) optical spectroscopic redshifts. The ASPECS CO−selected galaxies cover a
larger range of star formation rates (SFRs) and stellar masses compared to literature CO emitting galaxies at z > 1
for which scaling relations have been established previously. Most of ASPECS CO-selected galaxies follow these
established relations in terms of gas depletion timescales and gas fractions as a function of redshift, as well as the
SFR–stellar mass relation (“galaxy main sequence”). However, we find that ∼30% of the galaxies (5 out of 16) are
offset from the galaxy main sequence at their respective redshift, with ∼12% (2 out of 16) falling below this
relationship. Some CO-rich galaxies exhibit low SFRs, and yet show substantial molecular gas reservoirs, yielding
long gas depletion timescales. Capitalizing on the well-defined cosmic volume probed by our observations, we
measure the contribution of galaxies above, below, and on the galaxy main sequence to the total cosmic molecular
gas density at different lookback times. We conclude that main-sequence galaxies are the largest contributors to the
molecular gas density at any redshift probed by our observations (z ∼ 1−3). The respective contribution by
starburst galaxies above the main sequence decreases from z ∼ 2.5 to z ∼ 1, whereas we find tentative evidence
for an increased contribution to the cosmic molecular gas density from the passive galaxies below the main
sequence
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