Evolution of Galaxy Types and HI Gas Contents in Galaxy Groups
  Using the group crossing time $t_{\rm c}$ as an age indicator for galaxy
groups, we have investigated the correlation between $t_{\rm c}$ and the group
spiral fraction, as well as between $t_{\rm c}$ and the neutral hydrogen gas
fraction of galaxy groups. Our galaxy group sample is selected from the SDSS
DR7 catalog, and the group spiral fraction is derived from the Galaxy Zoo
morphological data set. We found that the group spiral galaxy fraction is
correlated with the group crossing time. We further cross matched the latest
released ALFALFA 70\% HI source catalog with the SDSS group catalog and have
identified 172 groups from the SDSS survey whose total HI mass can be derived
by summing up the HI mass of all the HI sources within the group radius. For
the galaxies not detected in the ALFALFA, we estimate their HI masses based on
the galaxies' optical colors and magnitudes. Our sample groups contain more
than 8 member galaxies, they cover a wide range of halo masses and are
distributed in different cosmic environments. We derived the group HI mass
fraction which is the ratio of group HI mass to the group virial mass. We found
a correlation between the HI mass fraction and the group crossing time. Our
results suggest that long time scale mechanisms such as starvation seem to play
a more important role than short time scale processes like stripping in
depleting HI gas in the SDSS galaxy groups.
