A Spectroscopically Confirmed Excess of \mipsmu Sources in a Super Galaxy Group at : 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 \mipsmu sources in a super galaxy group to the field and a rich galaxy cluster at . We draw on multi-wavelength observations999Based 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 , , and imaging with extensive optical spectroscopy ( redshifts) to isolate galaxies in each environment and thus ensure a uniform analysis. We focus on the four galaxy groups ( km s) 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 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 ([erg s]) not found in galaxy clusters at ; and 3) there is a strong trend of decreasing \mipsmu fraction with increasing galaxy density, 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, the timescales for morphological transformation cannot be strongly coupled to when the star formation is completely quenched. The supergroup has a significant fraction (%) of luminous, low-mass (), SFR yr members that are outside the group cores ( 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 yr galaxies is comparable in both environments, and 2) the supergroup’s IR luminosity function has a higher than that previously measured for the field.
Subject headings:galaxies: evolution – galaxies: starburst – galaxies: luminosity function, mass function – galaxies: clusters: general – galaxies: clusters: individual (SG1120-1202) – infrared: galaxies
Galaxies in the field environment span a wide range in morphology, color, and ongoing star formation ( Marzke et al. 1998). In contrast, the significantly more crowded environment of galaxy clusters is dominated by passive, red, early-type galaxies that formed the bulk of their stars at (Gregory & Thompson 1978; Dressler 1980; Bower et al. 1992; van Dokkum et al. 1998a). Using spectroscopically defined samples, several studies show that galaxies in clusters differ from their field counterparts even up to ( Holden et al. 2007; Mei et al. 2009; Lidman et al. 2008).
However, we know that the galaxy populations in clusters have evolved since at least . Observational examples of how the galaxy mix in clusters evolves with increasing redshift include: the increasing fraction of blue/star-forming members (Butcher & Oemler 1978; Ellingson et al. 2001); the increasing fraction of spectroscopically confirmed \mipsmu sources (Saintonge et al. 2008, Paper I); the increasing fraction of massive post-starburst members (Tran et al. 2003; Poggianti et al. 2004); the increasing fraction of active galactic nuclei (Eastman et al. 2007; Kocevski et al. 2009); the increasing fraction of star-forming galaxies with increasing galaxy density at , a reversal of what is observed at (Elbaz et al. 2007; Cooper et al. 2008); the decreasing fraction of S0 galaxies (Postman et al. 2005; Moran et al. 2007); and the decreasing fraction of faint red galaxies (; Tanaka et al. 2007; Stott et al. 2007; De Lucia et al. 2007, cf. Crawford et al. 2009).
The problem is that we have yet to identify clearly the physical mechanisms responsible for the dramatically different galaxy populations in clusters versus the field, nor the timescales needed for these mechanisms to operate. Although a plethora of physical processes have been invoked to quench star formation and transform galaxies into spheroidal systems, starvation (Bekki et al. 2002), ram-pressure stripping (Abadi et al. 1999), and galaxy harassment (Moore et al. 1998), stringent observational tests of when star formation is quenched and whether quenching is coupled to morphological transformation are needed to assess the relative importance of the physics at work. Simulations are sufficiently advanced that new insight can be obtained by, comparing star formation and gas-loss rates as a function of local density to the observations (Tonnesen et al. 2007).
Also, instead of focusing on massive clusters, the key to understanding the interplay between galaxy evolution and environment is to study galaxy groups because: 1) most galaxies in the local universe are in groups ( Geller & Huchra 1983); and 2) hierarchical structure formation predicts that galaxy clusters assemble from the merger and accretion of smaller structures, groups (Peebles 1970). Simulations show that the physical mechanisms normally associated with galaxy clusters are also effective in groups (Hester 2006; Kawata & Mulchaey 2008; McCarthy et al. 2008), but that they operate in groups at lower redshifts (Romeo et al. 2008).
In fact, the galaxy groups in the local universe do have more in common with galaxy clusters than with the field population, higher early-type fractions and lower mean star formation rates than the field (Zabludoff & Mulchaey 1998; Hashimoto et al. 1998; Tran et al. 2001; Blanton & Berlind 2007; Rasmussen et al. 2008). With the advent of large spectroscopic studies such as CNOC (Yee et al. 1996) and SDSS (Abazajian et al. 2003), the galaxy populations in groups can now be studied for statistically large samples ( Yang et al. 2007), and at intermediate redshifts ( Poggianti et al. 2008; Gal et al. 2008; Knobel et al. 2009). Although still nascent, spectroscopic studies of galaxy groups at find that the groups already have high early-type fractions (Jeltema et al. 2007; Wilman et al. 2008). However, whether star formation in groups is enhanced or simply quenched relative to the field is debated (Poggianti et al. 2009; Balogh et al. 2009).
The question then is whether the evolution of galaxies in clusters is driven primarily on group or on cluster scales. Our discovery of a supergroup of galaxies at allows us to uniquely answer this question. The supergroup (hereafter SG1120) is composed of multiple galaxy groups that we have shown will merge into a cluster comparable in mass to Coma by (Gonzalez et al. 2005), unlike the majority of clusters studied at that are too massive to be Coma progenitors. Because we know the galaxies in SG1120 will evolve into a cluster population, we can test whether the group galaxies are already like those in clusters. First results from our multi-wavelength study of SG1120 show that the group galaxies are in transition: SG1120 has a high fraction of early-type members (Kautsch et al. 2008), yet several of the most massive group galaxies are growing by dissipationless merging at (Tran et al. 2008).
Here we focus on the dust-obscured star formation in the supergroup as measured with MIPS (Rieke et al. 2004) \mipsmu observations. Studies find a surprising number of mid-infrared sources at cluster and group densities at (Elbaz et al. 2007; Bai et al. 2007; Koyama et al. 2008; Dressler et al. 2009), but this may be due to the general increase in the fraction of mid-infrared galaxies with redshift (Le Floc’h et al. 2005). To determine if there is an excess of IR sources in the galaxy groups making up SG1120, we compare the \mipsmu members in SG1120 to their counterparts in both the field and cluster environment at the same redshift.
For the cluster environment, we use CL 1358+62, a massive, dynamically evolved, X-ray luminous galaxy cluster at with a line-of-sight velocity dispersion of km s (Fisher et al. 1998, hereafter F98). Most of the 232 spectroscopically confirmed members are passive, early-type galaxies (van Dokkum et al. 1998b). In addition to the spectroscopy, we have HST/WFPC2 mosaics taken in F606W and F814W covering , and MIPS \mipsmu imaging.
The field sample is drawn from extensive observations of two higher redshift clusters: MS 2053–04 (; Tran et al. 2005) and MS 1054–03 (; Tran et al. 2007). In the combined area of approximately , we have measured spectroscopic redshifts for nearly 300 field galaxies at . These fields were also imaged with HST/WFPC2 in F606W and F814W, and with MIPS at \mipsmu. The depth and uniformity of our spectroscopic and photometric observations in these fields makes for a unique dataset that enables robust comparison across environment at .
Our study of how \mipsmu galaxies vary across environment is Paper II in our SMIRCS (/MIPS Infra-Red Cluster Survey) series and complements Paper I (Saintonge et al. 2008) where we explored how the \mipsmu fraction increases with increasing redshift in massive galaxy clusters at (; see also Finn et al., in prep). Throughout the paper, we use km s Mpc, , and ; at , this corresponds to a scale of 5.12 kpc per arcsec and a lookback time of 4 Gyr. All restframe magnitudes are in the Vega system.
At these redshifts (), our large number of spectroscopically confirmed group (174) and cluster members (232) combined with multi-wavelength imaging that includes MIPS observations is unique among existing surveys. Unlike many spectroscopic cluster surveys at intermediate redshifts, we do not select by optical color which can bias a sample towards members that are already on the red sequence, against blue, star-forming members. The depth of our redshift surveys in the cluster fields also enables us to identify a sample of field galaxies (; ) that have been observed and analyzed in the same manner as the group and cluster galaxies. The uniformity of our observations allows us to compare directly galaxy populations across a range of environments at .
2.1. Optical Imaging
2.1.1 Supergroup 1120 ()
The four X-ray luminous galaxy groups in the supergroup 1120-12 (hereafter SG1120) extend across an approximately region (Fig. 1). Optical photometry of the group galaxies is measured from VLT/VIMOS (LeFevre et al. 2003) mosaics in (; (PSF)), Magellan/LDSS3 mosaics in (; (PSF)), and a 10 pointing mosaic taken with HST/ACS in F814W (; /pixel). Near-infrared imaging was also obtained with KPNO/FLAMINGOS101010FLAMINGOS was designed and constructed by the IR instrumentation group (PI: R. Elston) at the University of Florida, Department of Astronomy, with support from NSF grant AST97-31180 and Kitt Peak National Observatory. and provides a mosaic (; PSF). The wide-field mosaics are generated with scamp and swarp111111http://astromatic.iap.fr (Bertin et al. 2002; Bertin 2006) which corrects the astrometry across the wide field and stitches the pointings together.
Line-matched photometric catalogs were generated using the VIMOS mosaic as the master detection image (SExtractor v2.5.0; Bertin & Arnouts 1996). While several close galaxy pairs (separation) are considered single objects in the catalog, this is appropriate for our analysis given that the same close pairs are also blended sources in the MIPS catalog (see §2.4). We use k-correct v4.1 (Blanton & Roweis 2007) to determine rest-frame absolute magnitudes (Vega) and K-corrections. As input, we use the MAGAUTO photometry from the imaging and assumed minimum photometric uncertainties in each bandpass of 0.05 mag. The photometry has been corrected for foreground Galactic extinction using the Schlegel et al. (1998) dust maps and the O’Donnell (1994) Milky Way extinction curve, assuming .
For consistency and to thus ensure that our comparisons are robust across the supergroup, cluster, and field samples, we calculate stellar masses in the same manner by following the prescription in Bell et al. (2003). Here mass-to-light ratios are calculated from colors using:
and a diet Salpeter initial mass function (IMF) is assumed. We use the diet Salpeter IMF defined in Bell & de Jong (2001) as having below and so the stellar mass using a diet Salpeter IMF is 70% of that for a regular Salpeter IMF (Salpeter 1955). Using an absolute magnitude for the sun of 121212http://www.ucolick.org/cnaw/sun.html, a galaxy with and has a stellar mass of .
2.1.2 Cl 1358+62 ()
The galaxy populations in CL 1358+62 (; F98) have been studied extensively using optical imaging and spectroscopy. For the cluster galaxies, we use the optical photometry measured by Holden et al. (2007, hereafter H07) from the HST/WFPC2 mosaics (total area of ). To summarize, a Sérsic profile () was fit to the surface brightness distribution in the HST/WFPC2 imaging of each spectroscopically confirmed member; over 85% of the cluster members have . Galaxy colors were determined from fluxes measured within a half-light radius; the half-light radii were determined using the F814W imaging. Note that at , the redshifted and filters are well-matched to F606W and F814W. As in SG1120, we convert the observed fluxes (corrected for Galactic extinction) to rest-frame magnitudes using , and estimate stellar masses using Eq. 1. For more details about the photometry and testing the robustness of the stellar mass determination for the cluster galaxies, we direct the reader to the extensive dicussion in H07.
2.1.3 Field Galaxies ()
Our field sample is drawn from a larger program that focused on galaxies in X-ray luminous clusters at intermediate redshifts. To select field galaxies in the same redshift range as SG1120 and CL1358, we use observations of galaxy clusters MS 2053–04 (; Tran et al. 2005) and MS1054–03 (; Tran et al. 2007). Both galaxy clusters were imaged by HST/WFPC2 in the F606W and F814W filters; each image mosaic is composed of six overlapping pointings and each mosaic covers an area of . The image reduction and photometry are detailed for MS2053 and MS1054 in Hoekstra et al. (2002) and van Dokkum et al. (2000), respectively.
Photometric catalogs were generated using SExtractor (see Tran et al. 2004) and we used to convert observed fluxes (measured with MAG_AUTO and corrected for Galactic extinction) to rest-frame magnitudes. As in the cases of the supergroup and cluster galaxies, stellar masses for the field galaxies are estimated using Eq. 1.
2.2. Optical Spectroscopy
2.2.1 Supergroup 1120 ()
The spectroscopic survey of the SG1120 field was completed using VLT/VIMOS (in 2003), Magellan/LDSS3 (in 2006), and VLT/FORS2 (in 2007; Appenzeller et al. 1998). The medium resolution spectroscopy corresponds to 2.5Å pix (VIMOS), 0.7Å pix (LDSS3), and 1.65Å pix (FORS2). Targets for the VIMOS masks were selected using mag, and targets for the later runs selected using mag. A total of 16 slit-masks were observed at varying position angles, thus our spectroscopic completeness is not affected by slit collisions.
Spectra from all of the observing runs were reduced using a combination of IRAF131313IRAF is distributed by the National Optical Astronomy Observatories. routines and custom software provided by D. Kelson (Kelson et al. 2000); see Tran et al. (2005) for further details on the spectral reductions. Redshifts were determined using IRAF cross-correlation routines, and each assigned redshift was visually compared to the 1D spectrum. Each redshift was then given a quality flag where corresponded to definite, probable, and maybe (single emission line). The spectral range for most of the supergroup members covers [OII] to [OIII].
The spectroscopic completeness in the HST/ACS footprint is shown in Fig. 2. Due to the supergroup’s elongated structure (see Fig. 1), spectra for a few of the bright () galaxies have not been obtained; however, these are foreground galaxies. The brightest group galaxy has mag, and the survey remains % complete to mag. For red supergroup members, the adopted magnitude limit used in our analysis of mag (see §2.2.2) corresponds to mag.
In the larger region centered on the HST/ACS mosaic, we have obtained spectra of 603 unique objects. Guided by breaks in the redshift distribution, we define group members to be at (Fig. 3). Considering only galaxies with redshift quality flag of gives 174 supergroup members. Four of the five X-ray luminous regions correspond to galaxy groups at while the fifth is a galaxy cluster at (Gonzalez et al. 2005). The coordinates, mean redshifts, and line-of-sight velocity dispersions of the individual groups are listed in Table 1, and Fig. 1 shows the spatial distribution of members on the HST/ACS mosaic.
2.2.2 Cl 1358+62 ()
A complete description of the spectroscopic survey in CL1358 including target selection, spectral reduction, wavelength calibration, sky subtraction, etc., is presented by F98. To summarize, WHT and MMT spectroscopy targeted objects with mag over a region; at this magnitude limit, the spectroscopic survey is % complete and not dependent on color (see F98, Fig. 2). The magnitude limit corresponds approximately to , and we use this limit for our luminosity-selected samples. For reference, the Coma cluster has 141414Here we use (Abell 1977) and a distance modulus of (Baum et al. 1997) for the Coma cluster.
From nearly 400 redshifts, cluster membership was confirmed for 232 galaxies; in our analysis, we consider only the 171 members that fall on the HST/WFPC2 mosaic (total area of ) that were studied by H07.
2.2.3 Field Galaxies ()
As part of our program on galaxy clusters at intermediate redshift, we also obtained redshifts for a large sample of field galaxies. Spectroscopic targets were selected using a magnitude cut of and in the MS2053 and MS1054 fields respectively. These magnitude-limited spectroscopic surveys were completed with Keck/LRIS (Oke et al. 1995) and resulted in a total of over 800 redshifts in the two fields; excluding the cluster members and considering only redshifts with provides 295 field galaxies at . Further observational details for each field are in Tran et al. (2004). Notably, the spectroscopic completeness in both cluster fields is % at (see Figs. 2 in Tran et al. 2005, 2007).
To ensure that we are observing the field galaxies at the same epoch as the group and cluster galaxies, we use only the field galaxies at (; Fig. 4). In this redshift range, we have 87 field galaxies; applying a magnitude () or mass () selection decreases the field sample to 28 and 21 galaxies respectively (see Table 2). Note that both galaxy clusters MS2053 (; Tran et al. 2005) and MS1054 (; Tran et al. 2007) are at higher redshift, thus our field sample is not contaminated with cluster galaxies.
2.3. Hubble Type
We have visually classified Hubble types that were assigned using HST imaging for % (371/401) of the spectroscopically-defined galaxy sample across all three environments; at , an even higher fraction (97%; 225/232) of the galaxies are classified. The high resolution HST imaging allows us to easily separate bulge vs. disk-dominated galaxies, and even to distinguish between elliptical and S0s (Postman et al. 2005). We use a simplified Hubble scheme where T-types are assigned to elliptical (), S0 (), spiral+irregular (), and merging () galaxies.
In SG1120, we use the T-types assigned by Kautsch et al. (2008) to 142 of the 143 group galaxies that fall on the HST/ACS mosaic. The galaxy groups in SG1120 have velocity dispersions that are significantly lower than in massive clusters such as CL1358 ( km s vs. 1027 km s; see Table 1), yet the groups are already dominated by early-type members: SG1120’s early-type fraction of % is already comparable to that of galaxy clusters at intermediate redshifts (Kautsch et al. 2008).
For the cluster (CL1358) and field galaxies, we have T-types assigned by D. Fabricant, M. Franx, and P. van Dokkum using HST/WFPC2 imaging (Fabricant et al. 2000). This team classified all galaxies in the cluster fields brighter than ; these classifications have been published in vD98, van Dokkum et al. (2000), and Tran et al. (2005). From this database, 161 of the 171 CL1358 galaxies in H07’s sample and 67 of the 87 field galaxies () have visual classifications.
2.4. Mips \mipsmu Imaging
Deep wide-field \mipsmu imaging of all the fields presented in our study was taken with MIPS (Rieke et al. 2004). We briefly summarize here the procedure for retrieving, reducing, and analyzing the MIPS imaging; further details are in Paper I. For SG1120, we retrieved the MIPS \mipsmu data sets from the archive and corrected the individual frames with the scan mirror position-dependent flats before combining the frames with the MOPEX software to a pixel size of . The integration time per pixel was 1200 seconds and the background level 35.5 MJy/sr.
With the large SG1120 MIPS mosaic (), we were able to determine a good point spread function (PSF) and thus measure \mipsmu fluxes via profile fitting. As a check, we compared the fluxes measured via profile fitting to aperture fluxes and found the values to be consistent; for the latter, we used an aperture diameter of as a compromise between maximizing the flux and minimizing contamination from close neighbors, and applied corrections based on fluxes derived from modeled PSFs (see Paper I). We matched the centroid position of the MIPS sources to the master -band catalog. We estimated the completeness of the SG1120 \mipsmu catalog by adding 50 sources modeled on the empirical PSF to the mosaic and repeating this process 20 times.
To convert the \mipsmu fluxes to star formation rates, we determined the total infrared luminosity () for each galaxy using a family of infrared spectral energy distributions (SEDs) from Dale & Helou (2002). Using the range of SEDs that are representative of galaxies in the Spitzer Infrared Nearby Galaxies Survey (Dale et al. 2007), we adopt the median conversion factor from to at where the SEDs give essentially the same values and the error due to the adopted conversion factor is only %. Combining this conversion with the completeness simulations, we estimate that the SG1120 \mipsmu catalog is 80% complete to [erg s]=43.8; this corresponds to a \mipsmu flux of approximately 105Jy and an IR star formation rate of SFR yr (Rieke et al. 2009).
For the smaller cluster fields, we followed essentially the same procedure except that we used APEX to measure fluxes within a diameter aperture and corrected the measured fluxes using the PSF from the SG1120 mosaic. The total integration times and background levels in these mosaics vary, but the \mipsmu imaging is essentially confusion-limited in these fields (see Paper I). We estimated the completeness of the \mipsmu catalogs by adding 30 sources into each mosaic and repeated the process 20 times for each mosaic. The \mipsmu catalogs are deeper than in the SG1120 field, the CL1358 \mipsmu catalog is 80% complete to [erg s]=43.5.
In the following analysis, we consider only the 143 supergroup galaxies in SG1120 that fall on the HST/ACS mosaic, the 171 cluster galaxies in CL1358 with photometry measured by H07 from the HST/WFPC2 mosaics, and the 87 field galaxies at , all of which also have HST/WFPC2 imaging. We are thus assured of a uniformly selected sample and can directly compare results across the three environments.
Our \mipsmu imaging identifies all galaxies with obscured star-formation rates of yr or greater, regardless of galaxy mass. However, actively star-forming galaxies tend to have lower mass-to-light ratios than galaxies dominated by older stars, galaxies on the red sequence, thus an optical luminosity-selected sample is likely to differ from a mass-selected sample. For this reason, we use both luminosity and mass-selected samples in our analysis to check the robustness of our results. Note that in fitting the infrared luminosity functions, we select based on total IR luminosity as determined with the \mipsmu fluxes.
3.1. Fraction of \mipsmu Sources
We first apply a luminosity limit set by the spectroscopic completeness in the CL1358 field (see §2.2) and consider only galaxies with absolute -band (Vega) magnitude brighter than ; due to the mixed galaxy population in our samples, we do not correct for passive evolution. The color-magnitude (CM) diagram for the galaxies in all three environments is shown in Fig. 5. The slope of the CM relation each panel is from vD98 who measured the CM relation in CL1358 using the early-type members; the CM relation is normalized to the red sequence in CL1358. The color deviation from the CM relation is denoted as where blue galaxies are classically defined has having (Butcher & Oemler 1978).
In the luminosity-limited sample, the fraction of \mipsmu sources in the cluster is significantly lower than in the field (7%151515Given the small number statistics, we assume a binomial distribution to calculate the error on the fractions. vs. 36%; Table 2). However, we find that the fraction of \mipsmu sources in the supergroup (%) is comparable to the field and four times greater than in the cluster. Figure 6 shows HST/ACS images of the supergroup galaxies with SFR yr and .
Because ongoing star formation can increase a galaxy’s total optical luminosity and thus scatter lower-mass systems into the luminosity-selected sample, we compare our results to a mass-selected sample (Table 2). As in Paper I, we consider only galaxies with stellar masses greater than . Again, the fraction of \mipsmu sources in the supergroup is higher than in the cluster (19% vs. 5%; Table 2); however, the fraction in the supergroup is now only about half that of the field.
In applying a mass-cut, we discover that SG1120 has a significant number of members (17; see Fig. 6) that are bright (), mostly late-type galaxies with stellar masses of (). Once star formation is quenched in these systems, they will fade and redden, and most will have , they will populate the faint end of the red sequence
We have assumed that the \mipsmu emission is due to star formation but as many authors have noted ( Donley et al. 2008), there is a likely contribution from dust-enshrouded active galactic nuclei (AGN). However, we stress that AGN contamination does not impact our conclusions because the relative fraction in each environment is small. In a 70 ksec /ACIS image of the SG1120 field (Gonzalez et al. 2005), only 4 of the 143 group galaxies are detected as X-ray point sources. As for the cluster galaxies (), Martini et al. (2007) estimate that the AGN fraction in two clusters is less than 3%. The possible number of field AGN is equally low: using Donley et al. (2008)’s survey of IR-detected AGN, we estimate that only one of the field IR sources can be an AGN. Note that if we account for these estimates of the AGN fraction, the difference in the IR star-forming fraction between the supergroup and cluster environment only increases (% vs. % in the luminosity-selected samples).
3.2. Infrared Luminosity Function
To better quantify how the \mipsmu sources in the supergroup differ from their counterparts in the cluster and in the field, we compare the infrared luminosity functions (IR LFs) of the supergroup (SG1120) and cluster (CL1358) with published results from the field in Fig. 7. We correct the observations in both SG1120 and CL1358 for spectroscopic and \mipsmu incompleteness; the 80% completeness limit for the \mipsmu sources is [erg s] and 43.5 in the supergroup and galaxy cluster, respectively.
where we fix the faint-end slope and adopt the best-fit chi-square minimization method to determine and (Table 3). Because studies show the IR LF in general has a relatively large number of bright sources and is better described by a double-exponential function (Le Floc’h et al. 2005), we adopt their approach and also fit a double-exponential function:
where we fix the constants and to the values measured for the field IR LF, and minimize with chi-square again. Note that to determine the faint-end slope in either function, deeper IR observations are required (Bai et al. 2006).
The IR LFs in both the supergroup and galaxy cluster are well-fit by both a Schechter and a double-exponential function (Fig. 7) using different values for and (see Table 3). However, the density of IR sources in the supergroup is dramatically higher than in the cluster, especially at [erg s].
Perhaps the large difference is due to CL1358 being unusually deficient in IR sources. We test this by taking the IR LF determined from \mipsmu observations of galaxy clusters at (Bai et al. 2006) and evolve the IR LF to using the observed evolution in the field IR LF (Le Floc’h et al. 2005). Bai et al. (2007, 2008) find that and , as derived from \mipsmu observations, evolve in approximately the same manner in galaxy clusters and the field to , although see Muzzin et al. (2008) for an alternative result. The IR LF for CL1358 () is consistent with the evolved cluster IR LF (Fig. 7; long-dashed curve). However, the density of IR sources in the supergroup is times higher than the number predicted from the evolved IR LF at [erg s].
The IR LF in the supergroup also differs from the IR LF for the field measured by Le Floc’h et al. (2005). The best-fit double-exponential function to the group IR LF has a measurably larger compared to the value for field galaxies at (): vs. . In comparison, Bai et al. (2008) find a similar value for both local cluster and field galaxies. (Table 3). SG1120’s higher relative to even that measured for the field suggests that star formation is enhanced in the group environment.
To summarize, the number of IR sources in the galaxy groups that make up SG1120 is significantly higher than in CL1358, a rich galaxy cluster at , and includes a population of very bright IR sources ([erg s]) that are not found in CL1358 nor in lower redshift clusters. The higher value of in the supergroup compared to that measured for the field at also indicates that the IR sources in the supergroup differ from their counterparts in the field, that star formation likely is enhanced in the group environment.
3.3. Local Environment
Having established that the \mipsmu population in the supergroup (SG1120) is different from that in the cluster (CL1358) and likely also the field environment, we examine how the galaxy populations for the luminosity-selected samples () depend on local environment, how star formation rate relates to galaxy density (Balogh et al. 1998; Gómez et al. 2003). In addition to the IR-bright population, we separate galaxies into optically-defined absorption-line ([OII]Å) and emission-line ([OII]Å) systems. In the supergroup and cluster, we define the local galaxy density using the distance to the nearest spectroscopically confirmed neighbor; we note that the following results do not change if we use instead the nearest neighbor.
Figure 8 (top left) shows how the fraction of SFR yr members in the supergroup steadily increases with decreasing local density. The increasing fraction of emission-line members with decreasing mirrors the trend for IR members, and the absorption-line fraction changes accordingly. The trend of an increasing IR fraction with decreasing local density remains even if we apply higher IR star formation rate threshold of 5 yr. In contrast, the IR population in the cluster (top right) shows essentially no trend with local environment: the absorption-line population dominates throughout the range of local densities explored here ( gal Mpc)161616While the galaxy density in the cluster environment extends to gal Mpc, we consider only the range that overlaps with the galaxy groups.. These results are in line with Paper I where we also find an increase in \mipsmu members outside the cores of massive clusters ( kpc). Our results argue for a physical mechanism that quenches star formation before the members reach the group cores.
At the lowest galaxy densities, the fraction of IR members in the supergroup is higher than even in the field: considering all members with gal Mpc, the IR fraction increases to 49% (26/53) compared to the field value of 38% (Table 2). At these low galaxy densities, the higher fraction of IR members in the supergroup relative to the field, while statistically not significant, is consistent with the higher measured in the group environment (see Table 3).
If we now examine how the mass-selected () samples depend on local environment (Fig. 8, bottom panels), the trend of increasing \mipsmu fraction with decreasing galaxy density in the supergroup is weaker: the absorption-line population dominates in both the group and the cluster environment at gal Mpc. These results are consistent with H07 who find that evolution in the early-type fraction in massive clusters () is weaker when considering only galaxies with versus a luminosity-selected sample, thus galaxies with lower masses play a significant role in the observed evolution of the cluster galaxy population.
We find that the supergroup has a population of luminous, \mipsmu detected late-type members with stellar masses of () that are mostly outside the groups’ cores (see Figures 1 & 6), at lower galaxy densities. It is these galaxies that cause the strong observed trend of decreasing \mipsmu fraction with increasing galaxy density in the luminosity-selected sample. As noted in §3.1, most of these will fade and redden and can populate the faint end of the red sequence once their star formation is quenched.
To study how galaxies evolve, galaxies are usually separated into active/emission-line and passive/absorption-line systems with the goal of isolating the physical process that connects the two phases, removal of a galaxy’s gas halts its star formation and the galaxy evolves from an active system into a passive one (Abadi et al. 1999; Kawata & Mulchaey 2008). The high fraction of \mipsmu galaxies in the supergroup and the field means that obscured star formation (IR phase) is important for at least 30% of optically-selected galaxies in both these environments; the IR phase is likely to be as important in clusters given that clusters grow via the accretion of field and group galaxies (Peebles 1970). In the following, we examine the physical properties of the \mipsmu galaxies to better understand where the IR population fits into our current picture of galaxy evolution.
4.1. Morphologies of \mipsmu Galaxies
To determine what the typical morphology of a \mipsmu galaxy is across environment at , we separate the samples into late-type (; disk-dominated) and early-type (; bulge-dominated) systems. In both the field and supergroup, most (%) of the late-type galaxies are IR-detected; the supergroup even has a few bulge-dominated systems that are IR-detected171717While none of the eight early-types in the field have SFR yr, this is likely due to our relatively small field sample because Lotz et al. (2008) do find a number of early-type galaxies at with comparable IR luminosities.. In contrast, only % of the late-type galaxies in the cluster are IR-detected, and none of the cluster’s bulge-dominated members are IR-detected. These differences are true in both the luminosity and mass-selected samples (Table 2).
Comparing the \mipsmu galaxies in the supergroup to the field again strongly suggests a difference between the two environments. In the luminosity-selected samples, the supergroup and field have similar \mipsmu fractions. However, the supergroup has a much higher fraction of E/S0 members: the E/S0 fraction in the supergroup is % but it is only % in the field181818The E/S0 fraction in our field sample is consistent with the E/S0 fraction measured by Driver et al. (1998) for a significantly larger field sample. (Table 2). Several of the \mipsmu galaxies in the supergroup are in merging, disk-dominated systems (see Fig. 6), but only one of the massive dissipationless merging pairs (Tran et al. 2008) is detected at \mipsmu.
Despite having double the fraction of early-type galaxies compared to the field, the supergroup has a high \mipsmu fraction due to a population of luminous (), low-mass () late-type members with SFR yr (see §3.1, §3.3, & Table 2). Note that in the mass-selected sample, the \mipsmu fraction in the supergroup drops from % to % while the field fraction remains high (%). Our results show that the timescales for morphological evolution and quenching of star formation must differ (see also Finn et al., in prep).
4.2. Star Formation on the Red Sequence
Across all three environments, there are \mipsmu sources that are also on the optical red sequence (see Fig. 5); here we use the classical definition of the red sequence as galaxies with (Butcher & Oemler 1984). The fraction of red \mipsmu sources depends on environment: it is highest in the field (21%; 6/29), decreases in the supergroup (7%; 7/98), and is is lowest in the cluster (3%; 3/105). These results do not depend on whether we use the luminosity or mass-selected sample.
Our results appear to conflict with Gallazzi et al. (2008) who find that the fraction of red \mipsmu sources in the Abell 901/902 supercluster (; Gray et al. 2002) peaks at intermediate densities typical of cluster outskirts and galaxy groups191919Because Gallazzi et al. (2008) estimate local galaxy density differently, we cannot compare their values directly to Fig. 8. These authors use a spectroscopic sample supplemented with members selected with photometric redshifts.. However, the authors estimate the field contamination in their magnitude range can be as high as 20%, and our study shows that the fraction of red \mipsmu galaxies in the field is times higher than in the groups. By using a spectroscopically selected sample, we circumvent possible problems due to field contamination.
In A901/902, Wolf et al. (2009) find that the dusty red star-forming members are primarily spiral galaxies, and that this population mostly overlaps with the “optically passive” spirals (as defined by color). We find similar results: all of the red \mipsmu galaxies in the field and cluster are disk-dominated systems (), and most of the red \mipsmu galaxies (%) in the groups are spirals as well (see Fig. 6).
We note that neither optical colors nor optical spectroscopy reliably identifies dusty red  star-forming spirals: summing across environment, only (10/15) of the red \mipsmu galaxies have [OII]Å, one third of \mipsmu members on the red sequence show no significant [OII] emission. This result is in line with earlier studies, Moustakas et al. (2006), that show optical spectroscopy can severely underestimate the level of activity. On a related note, many \mipsmu galaxies can be strongly extincted with E(B-V) values as high as (Cowie & Barger 2008); once corrected for extinction, many of the \mipsmu galaxies would not lie on the red sequence.
4.3. Progenitors of Faint Red Galaxies
As the groups in SG1120 merge to form a galaxy cluster, how do the \mipsmu members impact the overall galaxy population? In Fig. 9, we plot specific star formation rates (defined as SFR yr divided by stellar mass) versus stellar mass for the \mipsmu galaxies in the supergroup, field, and cluster. Assuming the \mipsmu members maintain their current star formation rates, perhaps only five out of the 72 massive () group galaxies will double their stellar masses, virtually all of the massive galaxies that will end up in the cluster are already in place.
In our analysis, we have identified a considerable number of luminous () galaxies in the supergroup that have SFR yr and stellar masses below (17/98; Table 2); it is this population that contributes the most to the difference between the \mipsmu population in the supergroup and in the cluster. Fig. 9 shows that even if these group galaxies can maintain their current star formation rates, most (15/17; %) will still have stellar masses of at ; the current average stellar mass for all 17 galaxies is . Note that most of these members are at Mpc from their respective group cores (see Fig. 1).
We test our hypothesis that these galaxies can evolve into () red galaxies by comparing their stellar masses to the faint red galaxies in CL1358, our massive galaxy cluster. Following De Lucia et al. (2007), we define faint red galaxies as having luminosities of 202020Using , the corresponding Vega -band magnitudes are . and . The average stellar mass of the faint red galaxies in the cluster is ; this is comparable to the average stellar mass of the luminous, low-mass, SFR yr supergroup galaxies. Assuming their star formation is quenched by , these supergroup galaxies will fade and redden to lie on the CM relation in less than a Gyr (see models by Bruzual & Charlot 2003). Their younger luminosity-weighted ages relative to the more massive galaxies will be consistent with the observed age spread in the Coma cluster (Poggianti et al. 2001). We stress that the luminous, low-mass, SFR yr supergroup galaxies are likely to be only one of multiple progenitors of faint red galaxies.
To quantify how dust-obscurred star formation varies with environment, we compare galaxies in a super galaxy group to those in the field and in a massive cluster at using a rich multi-wavelength dataset that includes imaging from (optical), (X-ray), and (\mipsmu). The strength of our work relies on extensive optical spectroscopy in our fields: the magnitude-limited spectroscopic surveys yielded a total of over 1800 unique redshifts and enable us to securely identify field, supergroup, and cluster members. We focus on the four X-ray luminous galaxy groups at (SG1120-12) that will merge to form a galaxy cluster comparable in mass to Coma (Gonzalez et al. 2005); the groups have line-of-sight velocity dispersions of km s. To ensure robust comparison, we consider only field galaxies at () and confirmed members of the massive galaxy cluster CL1358+62 (, Fisher et al. 1998).
We find that the supergroup has a significantly higher fraction of dusty star-forming members than the massive galaxy cluster: in the luminosity-selected () samples, 32% of the supergroup members have SFR yr compared to only 7% of the cluster members. The supergroup’s infrared luminosity function confirms that the density of IR sources is dramatically higher in the groups compared to the cluster. The supergroup members also include bright IR sources ([erg s]) not found in galaxy clusters at .
When selected by luminosity, the supergroup members show a strong trend of decreasing \mipsmu fraction with increasing local galaxy density, an infrared-density relationship. This mirrors the trend in the optically active members (as defined by [OII] emission). In contrast, the fraction of \mipsmu sources in the massive cluster stays essentially zero at all densities.
Comparison to the mass-selected () samples reveals that the higher \mipsmu fraction and the IR-density relation in the supergroup is due primarily to a population of luminous (), lower-mass (), late-type members with SFR yr (%). Most of these members are outside of the group cores ( Mpc). Assuming their star formation is quenched in the next Gyr, these members will fade and redden by , and most will become fainter () galaxies on the color-magnitude relation. The physical mechanism that quenches their star formation must be effective outside the group cores, in lower density environments.
In the supergroup, the excess of \mipsmu sources, the number of very bright \mipsmu members, and the infrared-density relationship is surprising because the E/S0 fraction is already as high as in the cluster (% for luminosity-selected sample; Kautsch et al. 2008). No further morphological evolution is required to bring the morphological distribution of the groups in line with the high early-type fractions observed in local galaxy clusters. In other words, the timescale for morphological transformation must not be strongly coupled to when star formation is completely quenched.
Our analysis indicates that the \mipsmu population in the supergroup differs even from the field: 1) the supergroup’s IR luminosity function has a measurably higher than the field; and 2) the E/S0 fraction in the supergroup is twice that of the field, yet the \mipsmu fraction in both environments are comparable. If dusty star formation is enhanced in the supergroup relative to the field, our IR-density analysis suggests that it occurs at densities of gal Mpc. A larger field sample selected with the same criteria as in the supergroup and cluster is needed to answer this question securely.
Our study highlights the importance of understanding galaxy evolution on group scales. A significant fraction (%) of optically selected galaxies in both the supergroup and field at have dust-obscured star formation; the IR-phase must be as important in clusters because clusters grow by accreting galaxy groups and field galaxies. As demonstrated in recent simulations of galaxy groups ( Hester 2006; Romeo et al. 2008; McCarthy et al. 2008; Kawata & Mulchaey 2008), the physical mechanisms that affect star formation and induce morphological evolution are already well underway in the galaxy groups that make up SG1120. We will continue dissecting how these galaxies are transformed by using recently obtained IFU observations to map the kinematics and star formation of the \mipsmu members.
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|ID||aaCoordinates (J2000) of the brightest galaxy in each group.||bbMean redshift () and line-of-sight velocity dispersion (; km s) determined using galaxies () within 500 kpc of the brightest group galaxy; and are determined using the biweight and jackknife estimators (Beers et al. 1990), respectively.||bbMean redshift () and line-of-sight velocity dispersion (; km s) determined using galaxies () within 500 kpc of the brightest group galaxy; and are determined using the biweight and jackknife estimators (Beers et al. 1990), respectively.||bbMean redshift () and line-of-sight velocity dispersion (; km s) determined using galaxies () within 500 kpc of the brightest group galaxy; and are determined using the biweight and jackknife estimators (Beers et al. 1990), respectively.||ccX-ray temperatures (keV) from Gonzalez et al. (2005).|
|AllaaConsidering only spectroscopically confirmed members that fall on the HST imaging.||87||143||171|
|Late-typesbbLate-type (disk-dominated) galaxies have Hubble classification of and early-type (bulge-dominated) galaxies have .||18||37||21|
|Early-typesbbLate-type (disk-dominated) galaxies have Hubble classification of and early-type (bulge-dominated) galaxies have .||8||61||80|
|Late-typesbbLate-type (disk-dominated) galaxies have Hubble classification of and early-type (bulge-dominated) galaxies have .||12||16||18|
|Early-typesbbLate-type (disk-dominated) galaxies have Hubble classification of and early-type (bulge-dominated) galaxies have .||8||56||81|
|SchechteraaFor the Schechter profile, we set (Bai et al. 2006, 2007).||Group|
|SchechteraaFor the Schechter profile, we set (Bai et al. 2006, 2007).||Cluster|
|SchechteraaFor the Schechter profile, we set (Bai et al. 2006, 2007).||EvolvedbbIR LF measured in galaxy clusters (Bai et al. 2006) evolved to using the evolution measured in the field IR LF (Le Floc’h et al. 2005).||44.53||4.0|
|Double-exponentialccFor the double-exponential profile, we set and (Le Floc’h et al. 2005)||Group|
|Double-exponentialccFor the double-exponential profile, we set and (Le Floc’h et al. 2005)||Cluster|
|Double-exponentialccFor the double-exponential profile, we set and (Le Floc’h et al. 2005)||(Field)ddField measured by Le Floc’h et al. (2005) for galaxies at ; we do not include because it is normalized differently in the field and in clusters.|