ALMA measures rapidly depleted molecular gas reservoirs in massive
  quiescent galaxies at z~1.5
  We present ALMA CO(2-1) spectroscopy of 6 massive
(log$_{10}$M$_{\rm{*}}/\rm{M}_\odot>$11.3) quiescent galaxies at $z\sim1.5$.
These data represent the largest sample using CO emission to trace molecular
gas in quiescent galaxies above $z>1$, achieving an average 3$\sigma$
sensitivity of M$_{\rm{H_{2}}}\sim10^{10}\rm{M}_\odot$. We detect one galaxy at
4$\sigma$ significance and place upper limits on the molecular gas reservoirs
of the other 5, finding molecular gas mass fractions
M$_{\rm{H_{2}}}$/M$_{\rm{*}}$=f$_{\rm{H_{2}}}<2-6$% (3$\sigma$ upper limits).
This is 1-2 orders of magnitude lower than coeval star-forming galaxies at
similar stellar mass, and comparable to galaxies at $z=0$ with similarly low
sSFR. This indicates that their molecular gas reservoirs were rapidly and
efficiently used up or destroyed, and that gas fractions are uniformly low
($<$6%) despite the structural diversity of our sample. The implied rapid
depletion time of molecular gas (t$_{\rm{dep}}<0.6$ Gyr) disagrees with
extrapolations of empirical scaling relations to low sSFR. We find that our low
gas fractions are instead in agreement with predictions from both the recent
SIMBA cosmological simulation, and from analytical "bathtub" models for gas
accretion onto galaxies in massive dark matter halos
(log$_{10}M_{\rm{halo}}/\rm{M}_\odot\sim14$ at $z=0$). Such high mass halos
reach a critical mass of log$_{10}M_{\rm{halo}}/\rm{M}_\odot>12$ by $z\sim4$
that halt the accretion of baryons early in the Universe. Our data is
consistent with a simple picture where galaxies truncate accretion and then
consume the existing gas at or faster than typical main sequence rates.
Alternatively, we cannot rule out that these galaxies reside in lower mass
halos, and low gas fractions may instead reflect either stronger feedback, or
more efficient gas consumption.
