The Importance of AGN in an Assembling Galaxy Cluster

We present results from our multi-wavelength study of SG1120, a super galaxy group at z=0.37, that will merge to form a galaxy cluster comparable in mass to Coma. We have spectroscopically confirmed 174 members in the four X-ray luminous groups that make up SG1120, and these groups have velocity dispersions of sigma(1D)=303-580 km/s. We find that the supergroup has an excess of 24 micron members relative to CL1358+62, a rich galaxy cluster at z=0.33. SG1120 also has an increasing fraction of 24 micron members with decreasing local galaxy density, i.e. an infrared-density relation, that is not observed in the rich cluster. We detect nine of the group galaxies in VLA 1.4 Ghz imaging, and comparison of the radio to total infrared luminosities indicates that about 30% of these radio-detected members have AGN. The radio map also reveals that one of the brightest group galaxies has radio jets. We are currently analysing the 1.4 Ghz observations to determine if AGN can significantly heat the intrahalo medium and if AGN are related to the excess of 24 micron members.Comment: 4 page proceedings, "The Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters

Multi-Wavelength Properties of Barred Galaxies in the Local Universe. I: Virgo Cluster

We study in detail how the barred galaxy fraction varies as a function of luminosity, HI gas mass, morphology and color in the Virgo cluster in order to provide a well defined, statistically robust measurement of the bar fraction in the local universe spanning a wide range in luminosity (factor of ~100) and HI gas mass. We combine multiple public data-sets (UKIDSS near-infrared imaging, ALFALFA HI gas masses, GOLDMine photometry). After excluding highly inclined systems, we define three samples where galaxies are selected by their B-band luminosity, H-band luminosity, and HI gas mass. We visually assign bars using the high resolution H-band imaging from UKIDSS. When all morphologies are included, the barred fraction is ~17-24% while for morphologically selected discs, we find that the barred fraction in Virgo is ~29-34%: it does not depend strongly on how the sample is defined and does not show variations with luminosity or HI gas mass. The barred fraction depends most strongly on the morphological composition of the sample: when the disc populations are separated into lenticulars (S0--S0/a), early-type spirals (Sa--Sb), and late-type spirals (Sbc--Sm), we find that the early-type spirals have a higher barred fraction (~45-50%) compared to the lenticulars and late-type spirals (~22-36%). This difference may be due to the higher baryon fraction of early-type discs which makes them more susceptible to bar instabilities. We do not find any evidence of barred galaxies being preferentially blue.Comment: 13 pages, 14 figures. Submitted to Ap

A VLT/FORS2 Multi-Slit Search for Lyman-alpha Emitting Galaxies at z~6.5

We present results from a deep spectroscopic search in the 9150A atmospheric window for z~6.5 Lyman-alpha emitting galaxies using the VLT/FORS2. Our multi-slit+narrow-band filter survey covers a total spatial area of 17.6 sq. arcmin in four different fields and reaches fluxes down to 5x10^(-18) erg/s/cm^2 (7 sigma detection). Our detection limit is significantly fainter than narrow-band searches at this redshift and fainter also than the unlensed brightness of Hu et al.'s HCM6A at z=6.56, and thus provides better overlap with surveys at much lower redshifts. Eighty secure emission line galaxies are detected. However, based on their clear continuum emission shortward of the line or the presence of multiple lines, none of these can be Ly-alpha emission at z~6.5. Our null result of finding no z~6.5 Ly-alpha emitters suggests that the number density of Ly-alpha emitters with L>2x10^(42) erg/s declines by ~2 between z~3 and z~6.5.Comment: accepted by ApJ Letters (originally submitted June 11, 2004

Survival of Massive Star-forming Galaxies in Cluster Cores Drives Gas-Phase Metallicity Gradients : The Effects of Ram Pressure Stripping

Recent observations of galaxies in a cluster at z=0.35 show that their integrated gas-phase metallicities increase with decreasing cluster-centric distance. To test if ram pressure stripping (RPS) is the underlying cause, we use a semi-analytic model to quantify the "observational bias" that RPS introduces into the aperture-based metallicity measurements. We take integral field spectroscopy of local galaxies, remove gas from their outer galactic disks via RPS, and then conduct mock slit observations of cluster galaxies at z=0.35. Our RPS model predicts a typical cluster-scale metallicity gradient of -0.03 dex/Mpc. By removing gas from the outer galactic disks, RPS introduces a mean metallicity enhancement of +0.02 dex at a fixed stellar mass. This gas removal and subsequent quenching of star formation preferentially removes low mass cluster galaxies from the observed star-forming population. As only the more massive star-forming galaxies survive to reach the cluster core, RPS produces a cluster-scale stellar mass gradient of -0.05 log(M_*/M_sun)/Mpc. This mass segregation drives the predicted cluster-scale metallicity gradient of -0.03 dex/Mpc. However, the effects of RPS alone can not explain the higher metallicities measured in cluster galaxies at z=0.35. We hypothesize that additional mechanisms including steep internal metallicity gradients and self-enrichment due to gas strangulation are needed to reproduce our observations at z=0.35.Comment: 17 pages, 21 figures, accepted for publication Ap

Forming Early-type Galaxies in Groups Prior to Cluster Assembly

We study a unique proto-cluster of galaxies, the supergroup SG1120-1202. We quantify the degree to which morphological transformation of cluster galaxies occurs prior to cluster assembly in order to explain the observed early-type fractions in galaxy clusters at z=0. SG1120-1202 at z~0.37 is comprised of four gravitationally bound groups that are expected to coalesce into a single cluster by z=0. Using HST ACS observations, we compare the morphological fractions of the supergroup galaxies to those found in a range of environments. We find that the morphological fractions of early-type galaxies (~60 %) and the ratio of S0 to elliptical galaxies (0.5) in SG1120-1202 are very similar to clusters at comparable redshift, consistent with pre-processing in the group environment playing the dominant role in establishing the observed early-type fraction in galaxy clusters.Comment: 5 pages, 2 figures, 2 tables. Accepted for publication in ApJ Letter

The mass distribution in an assembling super galaxy group at z=0.37z=0.37

We present a weak gravitational lensing analysis of supergroup SG1120$-$1202, consisting of four distinct X-ray-luminous groups, that will merge to form a cluster comparable in mass to Coma at $z=0$. These groups lie within a projected separation of 1 to 4 Mpc and within $\Delta v=550$ km s$^{-1}$ and form a unique protocluster to study the matter distribution in a coalescing system. Using high-resolution {\em HST}/ACS imaging, combined with an extensive spectroscopic and imaging data set, we study the weak gravitational distortion of background galaxy images by the matter distribution in the supergroup. We compare the reconstructed projected density field with the distribution of galaxies and hot X-ray emitting gas in the system and derive halo parameters for the individual density peaks. We show that the projected mass distribution closely follows the locations of the X-ray peaks and associated brightest group galaxies. One of the groups that lies at slightly lower redshift ($z\approx 0.35$) than the other three groups ($z\approx 0.37$) is X-ray luminous, but is barely detected in the gravitational lensing signal. The other three groups show a significant detection (up to $5 \sigma$ in mass), with velocity dispersions between $355^{+55}_{-70}$ and $530^{+45}_{-55}$ km s$^{-1}$ and masses between $0.8^{+0.4}_{-0.3} \times 10^{14}$ and $1.6^{+0.5}_{-0.4}\times 10^{14} h^{-1} M_{\odot}$, consistent with independent measurements. These groups are associated with peaks in the galaxy and gas density in a relatively straightforward manner. Since the groups show no visible signs of interaction, this supports the picture that we are catching the groups before they merge into a cluster.Comment: 10 pages, 10 figures, accepted for publication by Astronomy & Astrophysic

Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS1054-03 (z=0.83)

We present follow-up spectroscopy of the galaxy cluster MS1054-03 (z=0.83) confirming that at least six of the nine merging galaxy pairs identified by van Dokkum et al. (1999) are indeed bound systems: they have projected separations of R_s<10 kpc and relative line-of sight velocities of dv<165 km/s. For the remaining three pairs, we were unable to obtain redshifts of both constituent galaxies. To identify a more objective sample of merging systems, we select bound red galaxy pairs (R_s<=30 kpc, dv<=300 km/s) from our sample of 121 confirmed cluster members: galaxies in bound red pairs make up 15.7+/-3.6% of the cluster population. The (B-K_s) color-magnitude diagram shows that the pair galaxies are as red as the E/S0 members and have a homogeneous stellar population. The red pair galaxies span a large range in luminosity and internal velocity dispersion to include some of the brightest, most massive members (L>L*, sigma>200 km/s); these bound galaxy pairs must evolve into E/S0 members by z~0.7. These results combined with MS1054's high merger fraction and reservoir of likely future mergers indicates that most, if not all, of its early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.Comment: accepted by ApJ Letters; high resolution version of Fig. 2 available at http://www.exp-astro.phys.ethz.ch/tran/outgoing/ms1054mgrs.ps.g

SG1120-1202: Mass-quenching as Tracked by UV Emission in the Group Environment at z = 0.37

We use the Hubble Space Telescope to obtain WFC3/F390W imaging of the supergroup SG1120-1202 at z=0.37, mapping the UV emission of 138 spectroscopically confirmed members. We measure total (F390W-F814W) colors and visually classify the UV morphology of individual galaxies as "clumpy" or "smooth." Approximately 30% of the members have pockets of UV emission (clumpy) and we identify for the first time in the group environment galaxies with UV morphologies similar to the jellyfish galaxies observed in massive clusters. We stack the clumpy UV members and measure a shallow internal color gradient, which indicates unobscured star formation is occurring throughout these galaxies. We also stack the four galaxy groups and measure a strong trend of decreasing UV emission with decreasing projected group distance ($R_{proj}$). We find that the strong correlation between decreasing UV emission and increasing stellar mass can fully account for the observed trend in (F390W-F814W) - $R_{proj}$, i.e., mass-quenching is the dominant mechanism for extinguishing UV emission in group galaxies. Our extensive multi-wavelength analysis of SG1120-1202 indicates that stellar mass is the primary predictor of UV emission, but that the increasing fraction of massive (red/smooth) galaxies at $R_{proj}$ < 2$R_{200}$ and existence of jellyfish candidates is due to the group environment.Comment: 11 pages, 9 figures, 2 tables, accepted to Ap

A Search for Young Stars in the S0 Galaxies of a Super-Group at z=0.37

We analyze Galaxy Evolution Explorer UV data for a system of four gravitationally bound groups at z = 0.37, SG1120, which is destined to merge into a Coma-mass cluster by z = 0, to study how galaxy properties may change during cluster assembly. Of the 38 visually classified S0 galaxies, with masses ranging from log (M *)[M ☉] ≈ 10-11, we detect only one in the near-UV (NUV) channel, a strongly star-forming S0 that is the brightest UV source with a measured redshift placing it in SG1120. Stacking the undetected S0 galaxies (which generally lie on or near the optical red sequence of SG1120) still results in no NUV/far-UV (FUV) detection (\u3c2σ). Using our limit in the NUV band, we conclude that for a rapidly truncating star formation rate, star formation ceased at least ~0.1-0.7 Gyr ago, depending on the strength of the starburst prior to truncation. With an exponentially declining star formation history over a range of timescales, we rule out recent star formation over a wide range of ages. We conclude that if S0 formation involves significant star formation, it occurred well before the groups were in this current pre-assembly phase. As such, it seems that S0 formation is even more likely to be predominantly occurring outside of the cluster environment