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CAIRNS: The Cluster And Infall Region Nearby Survey III. Environmental Dependence of H-alpha Properties of Galaxies

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. Our long-slit spectroscopy yields estimates of star formation rates in environments from cluster cores to the general large-scale structure. The fraction of galaxies with current star formation in their inner disks as traced by H-alpha emission increases with distance from the cluster and converges to the ``field'' value only at 2-3 virial radii, in agreement with other investigations. However, among galaxies with significant current star formation (EW[Ha]geq2Å), there is no difference in the distribution of EW[Ha] inside and outside the virial radius. This surprising result, first seen by Carter et al., suggests that (1) star formation is truncated on either very short timescales or only at moderate and high redshifts or (2) that projection effects contaminate the measurement. The number density profiles of star-forming and non-star-forming galaxies indicate that, among galaxies projected inside the virial radius, at least half of the former and 20% of the latter are ``infall interlopers,'' galaxies in the infall region but outside the virial region. 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 from the field and excludes one model of the backsplash scenario of galaxy transformation. Finally, we quantify systematic uncertainties in estimating the global star formation in galaxies from their inner disks.