Molecular gas in a gravitationally lensed galaxy group at $z = 2.9$
  Most molecular gas studies of $z > 2.5$ galaxies are of intrinsically bright
objects, despite the galaxy population being primarily "normal" galaxies with
less extreme star formation rates. Observations of normal galaxies at high
redshift provide a more representative view of galaxy evolution and star
formation, but such observations are challenging to obtain. In this work, we
present ALMA $\rm ^{12}CO(J = 3 \rightarrow 2)$ observations of a
sub-millimeter selected galaxy group at $z = 2.9$, resulting in spectroscopic
confirmation of seven images from four member galaxies. These galaxies are
strongly lensed by the MS 0451.6-0305 foreground cluster at $z = 0.55$,
allowing us to probe the molecular gas content on levels of $\rm 10^9-10^{10}
\; M_\odot$. Four detected galaxies have molecular gas masses of $\rm
(0.2-13.1) \times 10^{10} \; M_\odot$, and the non-detected galaxies have
inferred molecular gas masses of $\rm < 8.0 \times 10^{10} \; M_\odot$. We
compare these new data to a compilation of 546 galaxies up to $z = 5.3$, and
find that depletion times decrease with increasing redshift. We then compare
the depletion times of galaxies in overdense environments to the field scaling
relation from the literature, and find that the depletion time evolution is
steeper for galaxies in overdense environments than for those in the field.
More molecular gas measurements of normal galaxies in overdense environments at
higher redshifts ($z > 2.5$) are needed to verify the environmental dependence
of star formation and gas depletion.
