AAS 205th Meeting, 9-13 January 2005
Session 119 Galaxy Clusters and Groups II
Oral, Wednesday, January 12, 2005, 10:00-11:30am, Pacific 2/3

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[119.06] Environmental Dependence of Galaxy H\alpha Properties

K. Rines (Yale/YCAA), M.J. Geller, M.J. Kurtz (SAO)

We investigate the environmental dependence of star formation in cluster virial regions and infall regions as part of CAIRNS (Cluster And Infall Region Nearby Survey), a large spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. As part of this survey, we obtained complete spectroscopic surveys of Ks limited samples in eight of the CAIRNS clusters. These clusters appear to be in different dynamical stages from major merger to gradual infall. Our long-slit spectroscopy yields estimates of star formation rates in environments from cluster cores to general large-scale structure. The fraction of galaxies with current star formation as traced by H\alpha emission increases with distance from the cluster and converges only at ~3 virial radii, in agreement with other investigations. However, among galaxies with significant current star formation ([EW(H\alpha)]> 2Å), there is no difference in the distribution of EW inside and outside the virial radius. This surprising result, first seen by Balogh et al., suggests either that star formation is truncated rapidly or that projection effects contaminate the measurement. We also show that the kinematics of star-forming galaxies in the infall region closely match those of absorption-dominated galaxies. This result shows that the star forming galaxies in the infall regions are not interlopers and excludes one model of environmental galaxy transformation.

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© 2004. The American Astronomical Society.