To trace how dust-obscured star formation varies with environment , we compare the fraction of \mipsmu sources in a super galaxy group to the field and a rich galaxy cluster at z \sim 0.35 .
We draw on multi-wavelength observations Based on observations made with 1 ) The ESO Telescopes at Paranal Observatories under program IDs 072.A-0367 , 076.B-0362 , 078.B-0409 ; 2 ) the NASA/ESA Hubble Space Telescope ( GO-10499 ) ; STScI is operated by the association of Universities for Research in Astronomy , Inc. under the NASA contract NAS 5-26555 ; 3 ) the Spitzer Space Telescope , which is operated by the Jet Propulsion Laboratory , California Institute of Technology under a contract with NASA ; support for this work was provided by NASA through an award issued by JPL/Caltech ( GO-20683 ) ; 4 ) the Chandra X-ray Observatory Center , which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060 ; and 5 ) the Magellan 6.5 m telescope operated by OCIW . 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 ( \sigma _ { 1 D } = 303 - 580 km s ^ { -1 } ) 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 } \geq 3 M _ { \odot } yr ^ { -1 } is four times higher than in the cluster ( 32 \pm 5 % vs. 7 \pm 2 % ) ; 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 ( \log ( L _ { IR } ) [ erg s ^ { -1 } ] > 45 ) not found in galaxy clusters at z \lesssim 0.35 ; and 3 ) there is a strong trend of decreasing \mipsmu fraction with increasing galaxy density , i . e . an infrared-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 can not be strongly coupled to when the star formation is completely quenched .
The supergroup has a significant fraction ( \sim 17 % ) of luminous , low-mass ( 10.0 < \log ( M _ { \ast } ) [ M _ { \odot } ] < 10.6 ) , SFR _ { IR } \geq 3 M _ { \odot } yr ^ { -1 } members that are outside the group cores ( R _ { proj } \geq 0.5 Mpc ) ; once their star formation is quenched , most will evolve into faint red galaxies .
Our analysis indicates that the supergroup ’ s \mipsmu 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 SFR _ { IR } \geq 3 M _ { \odot } yr ^ { -1 } galaxies is comparable in both environments , and 2 ) the supergroup ’ s IR luminosity function has a higher L _ { IR } ^ { \ast } than that previously measured for the field .