Quenching of star formation from a lack of inflowing gas to galaxies
  Star formation in half of massive galaxies was quenched by the time the
Universe was three billion years old. Very low amounts of molecular gas appear
responsible for this, at least in some cases, though morphological gas
stabilization, shock heating, or activity associated with accretion onto a
central supermassive black hole is invoked in other cases. Recent studies of
quenching by gas depletion have been based upon upper limits that are
insufficiently sensitive to determine this robustly, or stacked emission with
its problems of averaging. Here we report 1.3mm observations of dust emission
from six strongly lensed galaxies where star formation has been quenched, with
magnifications of up to a factor of 30. Four of the six galaxies are undetected
in dust emission, with an estimated upper limit on the dust mass of 0.0001
times the stellar mass, and by proxy (assuming a Milky Way molecular
gas-to-dust ratio) 0.01 times the stellar mass in molecular gas. This is two
orders of magnitude less molecular gas per unit stellar mass than seen in star
forming galaxies at similar redshifts. It remains difficult to extrapolate from
these small samples, but these observations establish that gas depletion is
responsible for a cessation of star formation in some fraction of high-redshift
galaxies.
