THE CIRCULAR VELOCITY FUNCTION OF GROUP GALAXIES

A robust prediction of Lambda CDM cosmology is the halo circular velocity function (CVF), a dynamical cousin of the halo mass function. The correspondence between theoretical and observed CVFs is uncertain, however: cluster galaxies are reported to exhibit a power-law CVF consistent with N-body simulations, but that of the field is distinctly Schechter-like, flattened compared to Lambda CDM expectations at circular velocities v(c) less than or similar to 200 km s(-1). Groups offer a powerful probe of the role environment plays in this discrepancy as they bridge the field and clusters. Here, we construct the CVF for a large, mass- and multiplicity-complete sample of group galaxies from the Sloan Digital Sky Survey. Using independent photometric vc estimators, we find no transition from field to Lambda CDM-shaped CVF above v(c) = 50 km s-1 as a function of group halo mass. All groups with 12.4 less than or similar to log M-halo/M-circle dot less than or similar to 15.1 (Local Group analogs to rich clusters) display similar Schechter-like CVFs marginally suppressed at low v(c) compared to that of the field. Conversely, some agreement with N-body results emerges for samples saturated with late-type galaxies, with isolated late-types displaying a CVF similar in shape to Lambda CDM predictions. We conclude that the flattening of the low-v(c) slope in groups is due to their depressed late-type fractions-environment affecting the CVF only to the extent that it correlates with this quantity-and that previous cluster analyses may suffer from interloper contamination. These results serve as useful benchmarks for cosmological simulations of galaxy formation.