SESSION IV: GALAXY GROUPS AND CLUSTERS

Sarah C. Gallagher

The University of Western Ontario

The handful of late-type galaxies that comprise Hickson Compact Group 31 are in the midst of ongoing and complex gravitational interactions, evocative of the process of hierarchical structure formation at high redshift. We explore the accuracy of this analogy through the investigation of modes of star formation from compact, parsec scale clusters to kiloparsec scale complexes. With sensitive, multicolor HST imaging, we characterize the large population of <10 Myr old star clusters that suffuse the system. From the examination of young and globular star cluster systems, we find that HCG 31 is representative of other interacting, actively star forming galaxies in the local Universe. However, in both the main galaxies and the tidal dwarf candidate F, complexes (sensitive to the magnitude of disk turbulence) have both sizes and masses more characteristic of z=1-2 galaxies. The low velocity dispersion of the system components, available reservoir of HI, and current star formation rate of ~4 solar masses per year, indicate that HCG 31 is likely to both exhaust its cold gas supply and merge within the next few Gyrs. We speculate that the end product will be a low-mass elliptical with an X-ray halo, a smaller version of the fossil groups found in the local Universe.

Jeffrey Kenney

Yale University

I discuss how the cluster environment has altered star formation in Virgo Cluster galaxies based on the Spitzer Survey Of Virgo (SPITSOV) and the H-alpha Survey Of Virgo (HATSOV). Through extensive optical, HI and radio continuum imaging, we now a clear idea of which galaxies are experiencing or have experienced ram pressure stripping, and which have likely experienced strong gravitational interactions. Ram pressure stripping of the ISM ends star formation within a well-defined stripping radius in the disk, and appears to be the main mechanism responsible for the low average star formation rates of Virgo spirals. In some of the galaxies experiencing ram pressure, extraplanar star formation is observed in the stripped gas. In others, modest localized starbursts are observed at the outer edges of the remaining gas disks, presumably triggered by ram pressure. Some galaxies with disturbed stellar disks due to tidal interactions have enhanced star formation in the central ~1 kpc, and no star formation beyond. Some have starburst outflows detected in H-alpha or 8 microns.

Matthew C. Fleenor

Roanoke College

We present specific indications of star-formation within groups, clusters, and intercluster filaments of the HRS (z = 0.05). These indicators arise from multi-wavelength observations, primarily emission lines from optical spectroscopy and faint excess from radio (1.4 and 2.5 GHz) photometry. HRS galaxies exhibiting current star formation are consistent with previous studies in that the star-forming populations organize around large-scale filament axes or infalling groups at cluster peripheries. Therefore, mechanisms for star formation in these galaxies are most likely due to galaxy-galaxy interactions and possible bursts prior to stripping of the local IGM due to cluster infall. We also characterize that faint dwarf galaxies are more susceptible to these processes.