0 Assigned ID: 2085 ----- Attempting Submission 1 (Sat Mar 02 00:55:59 GMT 2013) ----- While environmental effects on galaxies are seen clearly in dramatic environments like clusters and voids, most galaxies live in far more prosaic group environments of varying richness. Since typical groups consist of a relatively small handful of galaxies spread over a relatively large area, they have proven particularly difficult to detect and characterize up to high redshifts. With the Carnegie-Spitzer-IMACS (CSI) survey, we have developed the first large-scale, stellar mass-selected group catalog extending to z=1.2, and directly measured the hierarchical growth of groups therefrom. We are now embarking on a program to study in detail the influence of these groups on their galaxy populations through a combination of Magellan follow-up spectroscopy and deep public near-IR datasets. With the HST archival study proposed here, we will fold in the spectacular CANDELS and GOODS WFC3/ACS imaging in two of the CSI fields, tracking for the first time the morphological and structural transformations in ~30 stellar mass-selected groups (containing ~300-400 group galaxies) to z=1.2. UNRESOLVED STELLAR POPULATIONS AND GALAXY STRUCTURE Default Small false false false false false SUPPORTED