A Spectroscopically Confirmed Excess of 24 micron Sources in a Super
  Galaxy Group at z=0.37: Enhanced Dusty Star Formation Relative to the Cluster
  and Field Environment
  To trace how dust-obscured star formation varies with environment, we compare
the fraction of 24 micron sources in a super galaxy group to the field and a
rich galaxy cluster at z~0.35. We draw on multi-wavelength observations that
combine Hubble, Chandra, and Spitzer imaging with extensive optical
spectroscopy (>1800 redshifts) to isolate galaxies in each environment and thus
ensure a uniform analysis. We focus on the four galaxy groups in supergroup
1120-12 that will merge to form a galaxy cluster comparable in mass to Coma. We
find that 1) the fraction of supergroup galaxies with SFR(IR)>3 Msun/yr is four
times higher than in the cluster (32% vs. 7%); 2) the supergroup's infrared
luminosity function confirms that it has a higher density of IR members
compared to the cluster and includes bright IR sources not found in galaxy
clusters at z<0.35; and 3) there is a strong trend of decreasing IR fraction
with increasing galaxy density, i.e. an IR-density relation, not observed in
the cluster. These dramatic differences are surprising because the early-type
fraction in the supergroup is already as high as in clusters, i.e. the
timescales for morphological transformation cannot be strongly coupled to when
the star formation is completely quenched. The supergroup has a significant
fraction (~17%) of luminous, low-mass, IR members that are outside the group
cores (R>0.5 Mpc); once their star formation is quenched, most will evolve into
faint red galaxies. Our analysis indicates that the supergroup's 24 micron
population also differs from that in the field: 1) despite the supergroup
having twice the fraction of E/S0s as the field, the fraction of IR galaxies is
comparable in both environments, and 2) the supergroup's IR luminosity function
has a higher L(IR)* than that previously measured for the field.
