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Session 123 - Galaxy Evolution: Dwarfs & Clusters.
Oral session, Saturday, January 10
How does the environment affect the star formation histories of galaxies in clusters? This question is addressed through detailed analysis of R and H\alpha CCD images of 63 Virgo Cluster and 30 nearby isolated spiral galaxies. The spatial distributions of massive star formation and the old stellar population are derived from the images and compared as a function of environment. Quantitative parameters which trace the central continuum light concentration and the massive star formation activity are used to compare sample galaxies independent of the assigned Hubble types.
Many Virgo Cluster galaxies have reduced total star formation activity compared to isolated counterparts of similar Hubble type and concentration. The star formation in Virgo Cluster galaxies has been reduced primarily in the outer disk. The inner disks of Virgo Cluster galaxies have star formation rates similar to or enhanced with respect to isolated counterparts. The reduced outer disk star formation has caused a significant fraction of Virgo Cluster galaxies with small to intermediate central light concentrations to be assigned misleading early-type Hubble classifications. This has consequences for previous comparisons of cluster and field galaxies, including the morphology-density relationship. It is likely that the increasing fraction of early-type spiral galaxies in clusters is partially due to misleading Hubble classifications of galaxies with reduced star formation.
There is evidence for a variety of environmental effects in Virgo Cluster spiral galaxies, including ICM-ISM stripping, mergers, high-velocity penetrating collisions, and non-penetrating tidal encounters. The star formation histories of Virgo Cluster galaxies are thus influenced by both the removal and redistribution of gas.
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Program listing for Saturday