The Influence of Environment on the Star Formation Rates of Galaxies

We have used a sample of 15749 galaxies taken from the Las Campanas Redshift Survey (Shectman et al. 1996) to investigate the effects of environment on the rate of star formation in galaxies. For each galaxy we derive a measure of star formation rate (SFR) based on the strength of the [OII] emission line, and a measure of galactic structure based on the central concentration of the galaxy light, which is used to decouple the effect of “morphology-environment” relation from the SFR. Galactic environment is characterized both by the three-space local density of galaxies and by membership in groups and clusters. The size and homogeneity of this data set allows us to sample, for the first time, the entire range of galactic environment, from the lowest density voids to the richest clusters, in a uniform manner. Thus, we could expand our research from the conventional cluster vs. field comparison to a new “general” environmental investigation by decoupling the local galaxy density from the membership in associations. This decoupling is very crucial for constraining the physical processes responsible for the observed environmental dependencies of star formation. On the other hand, the use of an automatic measure of galactic structure (concentration index), rather than Hubble type which is subjective and star formation-contaminated estimate of galactic morphologies, allows us to cleanly separate the morphological component from the SFR vs. environment relationship.