CAIRNS:: The Cluster and Infall Region Nearby Survey.: III.: Environmental dependence of Hα properties of galaxies

We investigate the environmental dependence of star formation in cluster virial regions and infall regions as part of the Cluster and Infall Region Nearby Survey (CAIRNS), a large spectroscopic survey of the infall regions surrounding nine nearby rich clusters of galaxies. We use complete, homogeneous spectroscopic surveys of K(s)-limited samples in eight of the CAIRNS clusters. Our long-slit spectroscopy yields estimates of star formation rates in environments from cluster cores to the general large-scale structure. Galaxies in infall regions probe whether processes affecting star formation are effective over scales larger than cluster virial regions. 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 [H alpha] >= 2 angstrom), there is no difference in the distribution of EW[H alpha] inside and outside the virial radius. This surprising result, first seen by Carter and coworkers, suggests that (1) star formation is truncated either on very short timescales or only at moderate and high redshifts or (2) that projection effects contaminate the measurement. We quantify the possible impact of mechanisms that only affect the outer parts of galaxies and thus might not be detected in this survey or any fiber-based survey. The number density profiles of star-forming and non-star-forming galaxies indicate that, among galaxies projected inside the virial radius, at least half the former and 20% of the latter are infall interlopers, galaxies in the infall region but outside the virial region. We 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 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.