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To trace how dust-obscured star formation varies with environment, we comparethe fraction of 24 micron sources in a super galaxy group to the field and arich galaxy cluster at z~0.35. We draw on multi-wavelength observations thatcombine Hubble, Chandra, and Spitzer imaging with extensive opticalspectroscopy (>1800 redshifts) to isolate galaxies in each environment and thusensure a uniform analysis. We focus on the four galaxy groups in supergroup1120-12 that will merge to form a galaxy cluster comparable in mass to Coma. Wefind that 1) the fraction of supergroup galaxies with SFR(IR)>3 Msun/yr is fourtimes higher than in the cluster (32% vs. 7%); 2) the supergroup's infraredluminosity function confirms that it has a higher density of IR memberscompared to the cluster and includes bright IR sources not found in galaxyclusters at z<0.35; and 3) there is a strong trend of decreasing IR fractionwith increasing galaxy density, i.e. an IR-density relation, not observed inthe cluster. These dramatic differences are surprising because the early-typefraction in the supergroup is already as high as in clusters, i.e. thetimescales for morphological transformation cannot be strongly coupled to whenthe star formation is completely quenched. The supergroup has a significantfraction (~17%) of luminous, low-mass, IR members that are outside the groupcores (R>0.5 Mpc); once their star formation is quenched, most will evolve intofaint red galaxies. Our analysis indicates that the supergroup's 24 micronpopulation also differs from that in the field: 1) despite the supergrouphaving twice the fraction of E/S0s as the field, the fraction of IR galaxies iscomparable in both environments, and 2) the supergroup's IR luminosity functionhas a higher L(IR)* than that previously measured for the field.

astrophysical journal/the astrophysical journal

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