Evolution in the H I Gas Content of Galaxy Groups: Pre-Processing and
  Mass Assembly in the Current Epoch
  We present an analysis of the neutral hydrogen (HI) content and distribution
of galaxies in groups as a function of their parent dark matter halo mass. The
Arecibo Legacy Fast ALFA survey alpha.40 data release allows us, for the first
time, to study the HI properties of over 740 galaxy groups in the volume of sky
common to the SDSS and ALFALFA surveys. We assigned ALFALFA HI detections a
group membership based on an existing magnitude/volume-limited SDSS DR7
group/cluster catalog. Additionally, we assigned group "proximity" membership
to HI detected objects whose optical counterpart falls below the limiting
optical magnitude--thereby not contributing substantially to the estimate of
the group stellar mass, but significantly to the total group HI mass. We find
that only 25% of the HI detected galaxies reside in groups or clusters, in
contrast to approximately half of all optically detected galaxies. Further, we
plot the relative positions of optical and HI detections in groups as a
function of parent dark matter halo mass to reveal strong evidence that HI is
being processed in galaxies as a result of the group environment: as optical
membership increases, groups become increasingly deficient of HI rich galaxies
at their center and the HI distribution of galaxies in the most massive groups
starts to resemble the distribution observed in comparatively more extreme
cluster environments. We find that the lowest HI mass objects lose their gas
first as they are processed in the group environment, and it is evident that
the infall of gas rich objects is important to the continuing growth of large
scale structure at the present epoch, replenishing the neutral gas supply of
groups. Finally, we compare our results to those of cosmological simulations
and find that current models cannot simultaneously predict the HI selected halo
occupation distribution for both low and high mass halos.
