Environment and the cosmic evolution of star formation

We present a mark correlation analysis of the galaxies in the Sloan Digital Sky Survey using weights provided by MOPED. The large size of the sample permits statistically significant statements about how galaxies with different metallicities and star formation histories are spatially correlated. Massive objects formed a larger fraction of their stars at higher redshifts and over shorter timescales than did less massive objects (sometimes called downsizing). We find that galaxies in clusters today formed stars at above average rates at but have had z > 3 below average star formation rates for the last 5 Gyr. Hence, our results indicate that star formation and chemical enrichment have occurred at above average rates in the denser regions of the universe. Our conclusions about the environment did not require classification of the galaxies into cluster and field populations, and our conclusions about evolution did not require a galaxy sample that spans a wide redshift range.