Determining the Halo Mass Scale where Galaxies Lose Their Gas
  A major question in galaxy formation is how the gas supply that fuels
activity in galaxies is modulated by their environment. We use spectroscopy of
a set of well characterized clusters and groups at $0.4<z<0.8$ from the ESO
Distant Cluster Survey (EDisCS) and compare it to identically selected field
galaxies. Our spectroscopy allows us to isolate galaxies that are dominated by
old stellar populations. Here we study a stellar-mass limited sample
($\log(M_*/M_\odot)>10.4$) of these old galaxies with weak [OII] emission. We
use line ratios and compare to studies of local early type galaxies to conclude
that this gas is likely excited by post-AGB stars and hence represents a
diffuse gas component in the galaxies. For cluster and group galaxies the
fraction with EW([OII])$>5$\AA\ is $f_{[OII]}=0.08^{+0.03}_{-0.02}$ and
$f_{[OII]}=0.06^{+0.07}_{-0.04}$ respectively. For field galaxies we find
$f_{[OII]}=0.27^{+0.07}_{-0.06}$, representing a 2.8$\sigma$ difference between
the [OII] fractions for old galaxies between the different environments. We
conclude that a population of old galaxies in all environments has ionized gas
that likely stems from stellar mass loss. In the field galaxies also experience
gas accretion from the cosmic web and in groups and clusters these galaxies
have had their gas accretion shut off by their environment. Additionally,
galaxies with emission preferentially avoid the virialized region of the
cluster in position-velocity space. We discuss the implications of our results,
among which is that gas accretion shutoff is likely effective at group halo
masses (log~${\cal M}/$\msol$>12.8$) and that there are likely multiple gas
removal processes happening in dense environments.
