TRACING THE STAR-FORMATION-DENSITY RELATION TO z ∼ 2

Recent work has shown that the star formation (SF) density relation-in which galaxies with low SF rates are preferentially found in dense environments-is still in place at z similar to 1, but the situation becomes less clear at higher redshifts. We use mass-selected samples drawn from the UKIDSS Ultra-Deep Survey to show that galaxies with quenched SF tend to reside in dense environments out to at least z similar to 1.8. Over most of this redshift range we are able to demonstrate that this SF-density relation holds even at fixed stellar mass. The environmental quenching of SF appears to operate with similar efficiency on all galaxies regardless of stellar mass. Nevertheless, the environment plays a greater role in the buildup of the red sequence at lower masses, whereas other quenching processes dominate at higher masses. In addition to a statistical analysis of environmental densities, we investigate a cluster at z = 1.6, and show that the central region has an elevated fraction of quiescent objects relative to the field. Although the uncertainties are large, the environmental quenching efficiency in this cluster is consistent with that of galaxy groups and clusters at z similar to 0. In this work we rely on photometric redshifts and describe some of the pitfalls that large redshift errors can present.