What triggers galaxy transformations? The environments of poststarburst galaxies

There are good observational reasons to believe that the progenitors of these red galaxies have undergone starbursts, followed by a poststarburst phase. Poststarburst ("K+A'' or "E+A'') galaxies appear in the SDSS visible spectroscopic data by showing an excess of A-star light but relatively little H alpha emission. We investigate the environments of these galaxies by measuring (1) number densities in 8 h(-1) Mpc radius comoving spheres, (2) transverse distances to nearest Virgo-like galaxy clusters, and (3) transverse distances to nearest luminous-galaxy neighbors. We compare the poststarburst galaxies to currently star-forming galaxies identified solely by A-star excess or H alpha emission. We find that poststarburst galaxies are in the same kinds of environments as star-forming galaxies; this is our "null hypothesis.'' More importantly, we find that at each value of the A-star excess, the star-forming and poststarburst galaxies lie in very similar distributions of environment. Other studies finding similar results have argued that galaxy transformations occur slowly (timescales > 1 Gyr), but this is at odds with the evidence that red galaxies are formed via starbursts. The only deviations from our null hypothesis are barely significant: a slight deficit of poststarburst galaxies (relative to the star-forming population) in very low-density regions, a small excess inside the virial radii of clusters, and a slight excess with nearby neighbors. None of these effects is strong enough to make the poststarburst galaxies a high-density phenomenon, or to argue that the starburst events are primarily triggered by external tidal impulses from close passages. The small excess inside cluster virial radii suggests that some poststarbursts are triggered by interactions with the intracluster medium, but this represents a very small fraction of all poststarburst galaxies.