The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: Evolution
  of the molecular gas in CO-selected galaxies
  We analyze the interstellar medium properties of a sample of sixteen 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 a couple of additional CO line emitters in the UDF
that are identified based on their MUSE optical spectroscopic redshifts. The
ASPECS CO$-$selected galaxies cover a larger range of star-formation rates 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
star-formation rate-stellar mass relation (`galaxy main sequence'). However, we
find that $\sim30\%$ of the galaxies (5 out of 16) are offset from the galaxy
main sequence at their respective redshift, with $\sim12\%$ (2 out of 16)
falling below this relationship. Some CO-rich galaxies exhibit low
star-formation rates, 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 contributor to the molecular gas density at any redshift probed
by our observations (z$\sim$1$-$3). The respective contribution by starburst
galaxies above the main sequence decreases from z$\sim$2.5 to z$\sim$1, whereas
we find tentative evidence for an increased contribution to the cosmic
molecular gas density from the passive galaxies below the main sequence.
