Environment and Galaxy Evolution at Intermediate Redshift in the CNOC2 Survey

(abridged) The systematic variation of galaxy colors and types withclustering environment could either be the result of local conditions atformation or subsequent environmental effects as larger scale structures drawtogether galaxies whose stellar mass is largely in place. At z~0.4 theco-moving galaxy correlation length, r_0, measured in the CNOC2 sample isstrongly color dependent, rising from 2/h Mpc to nearly 10/h Mpc as thevolume-limited subsamples range from blue to red. The luminosity dependence ofr_0 at z~0.4 is weak below L_ast although there is an upturn at high luminositywhere its interpretation depends on separating it from the r_0-color relation.The dominant effect of the group environment on star formation is seen in theradial gradient of the mean galaxy colors which on the average become redderthan the field toward the group centers. The redder-than-field trend applies togroups with a line-of-sight velocity dispersion, sigma_1>150 kms. There is anindication, somewhat statistically insecure, that the high luminosity galaxiesin groups with sigma_1<125 kms become bluer toward the group center. Weconclude that the higher velocity dispersion groups largely act to suppressstar formation relative to the less clustered field, leading to ``embalmed''galaxies. The tidal fields within the groups appear to be a strong candidatefor the physical source of the reduction of star formation in group galaxiesrelative to field. Tides operate effectively at all velocity dispersions toremove gas rich companions and low density gas in galactic halos. Given thatmuch of the field population is in groups we suggest that this suppression maybe the dominant galaxy evolution force at low redshift.Comment: ApJ accepte