Program: GS-2011A-Q-30

We propose to complete our multi-semester redshift survey of galaxies in 16 groups at 0.8<z<1, selected from ultradeep X-ray observations. Achieving high completeness in each group, including galaxies an order of magnitude less massive than are typically included in wide-field surveys, allows us to answer important questions about how dense environments influence galaxy evolution. Only with such data is it possible to robustly measure the group mass, centre and radius, and the group-to-group variations predicted by simple infall models. Excellent data on half the sample (including >100 new members) have been obtained. A first paper has been submitted, in which we report the exciting discovery of a large population of confirmed group members "caught in the act" of transforming from the blue to red sequence. To conclusively demonstrate that these galaxies are associated with the group environment, however, requires the completion of the survey (to improve the statistical significance from 98% to 99.95%). Only with the full sample of 16 groups can we begin to address our main questions, about how this population (and other properties like galaxy formation efficiency) vary from group-to-group and perhaps correlate with other group properties, like X-ray emission.