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 clustervirial regions and infall regions as part of CAIRNS (Cluster And Infall RegionNearby Survey), a large spectroscopic survey of the infall regions surroundingnine nearby rich clusters of galaxies. Our long-slit spectroscopy yieldsestimates of star formation rates in environments from cluster cores to thegeneral large-scale structure. The fraction of galaxies with current starformation in their inner disks as traced by H-alpha emission increases withdistance from the cluster and converges to the ``field'' value only at 2-3virial radii, in agreement with other investigations. However, among galaxieswith significant current star formation (EW[Ha]geq2\AA), there is no differencein the distribution of EW[Ha] inside and outside the virial radius. Thissurprising result, first seen by Carter et al., suggests that (1) starformation is truncated on either very short timescales or only at moderate andhigh redshifts or (2) that projection effects contaminate the measurement. Thenumber density profiles of star-forming and non-star-forming galaxies indicatethat, among galaxies projected inside the virial radius, at least half of theformer and 20% of the latter are ``infall interlopers,'' galaxies in the infallregion but outside the virial region. The kinematics of star-forming galaxiesin the infall region closely match those of absorption-dominated galaxies. Thisresult shows that the star forming galaxies in the infall regions are notinterlopers from the field and excludes one model of the backsplash scenario ofgalaxy transformation. Finally, we quantify systematic uncertainties inestimating the global star formation in galaxies from their inner disks.Comment: 25 pages, 21 figures, accepted for publication in A