Stefan Kautsch

A SPECTROSCOPICALLY CONFIRMED EXCESS OF 24 μm 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 μm sources in a super galaxy group to the field and a rich galaxy cluster at z ~ 0.35. We draw on multi-wavelength observations9Based 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 (σ1D = 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 SFRIR ≥ 3 M sun yr–1 is 4 times higher than in the cluster (32% ± 5% versus 7% ± 2%); (2) the supergroups 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 lsim 0.35; and (3) there is a strong trend of decreasing 24 μm 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 cannot be strongly coupled to when the star formation is completely quenched. The supergroup has a significant fraction (~17%) of luminous, low-mass (10.0 < log(M *)[M sun] < 10.6), SFRIR ≥ 3 M sun yr–1 members that are outside the group cores (R proj ≥ 0.5 Mpc); once their star formation is quenched, most will evolve into faint red galaxies. Our analysis indicates that the supergroups 24 μm 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 SFRIR ≥ 3 M sun yr–1 galaxies is comparable in both environments, and (2) the supergroups IR luminosity function has a higher L*IR than that previously measured for the field.

The Late Stellar Assembly of Massive Cluster Galaxies via Major Merging

We present multiwavelength observations of the brightest galaxies in four X-ray-luminous groups at z ~ 0.37 that will merge to form a cluster comparable in mass to Coma. Ordered by increasing stellar mass, the four brightest group galaxies (BGGs) present a time sequence where BGG-1, 2, and 3 are in merging systems and BGG-4 is a massive remnant (M* = 6.7 × 1011 M☉). BGG-1 and 2 have bright, gravitationally bound companions and BGG-3 has two nuclei separated by only 2.5 kpc; thus, merging at z 3 Gyr, and their tight scatter in (B − V) color (σBV = 0.032) confirms that they formed the bulk of their stars at z > 0.9. Optical spectroscopy shows no signs of recent (< 1.5 Gyr) or ongoing star formation. Only two BGGs are weakly detected at 24 μm, and X-ray and optical data indicate that the emission in BGG-2 is due to an AGN. All four BGGs and their companions are early-type (bulge-dominated) galaxies, and they are embedded in diffuse stellar envelopes up to ~140 kpc across. The four BGG systems must evolve into the massive, red, early-type galaxies dominating local clusters. Our results show that (1) massive galaxies in groups and clusters form via dissipationless merging and (2) the group environment is critical for this process.

Forming Early-Type Galaxies in Groups Prior to Cluster Assembly

We study a unique protocluster 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 composed 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 preprocessing in the group environment playing the dominant role in establishing the observed early-type fraction in galaxy clusters.

Edge‐on disk galaxies in the SDSS DR6: Fractions of bulgeless and other disk galaxies

The aim of this study is to determine the fractions of different spiral galaxy types, especially bulgeless disks, from a complete and homogeneous sample of 15127 edge-on disk galaxies extracted from the sixth data release from the Sloan Digital Sky Survey. The sample is divided in broad morphological classes and sub types consisting of galaxies with bulges, intermediate types and galaxies which appear bulgeless. A small fraction of disky irregulars is also detected. The morphological separation is based on automated classification criteria which resemble the bulge sizes and the flatness of the disks. Each of these broad classes contains about 1/3 of the total sample. Using strict criteria for selecting pure bulgeless galaxies leads to a fraction of 15% of simple disk galaxies. We compare this fraction to other galaxy catalogs and find an excellent agreement of the observed frequency of bulgeless galaxies. Although the fraction of simple disk galaxies in this study does not represent a cosmic fraction of bulgeless galaxies, it shows that the relative abundance of pure disks is comparable to other studies and offers a profound value of the frequency of simple disks in the local Universe. This fraction of simple disks emphasizes the challenge for formation and evolution models of disk galaxies since these models are hard pressed to explain the observed frequency of these objects.