The Circular Velocity Function of Group Galaxies

A robust prediction of $Λ{\rm CDM}$ 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 relative to $Λ{\rm CDM}$ expectations at circular velocities $v_c \lesssim 200\, {\rm km\, s^{-1}}$. 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 $v_c$ estimators, we find no transition from a field- to $Λ{\rm CDM}$-shaped CVF above $v_c = 50\, {\rm km\, s^{-1}}$ as a function of group halo mass. All groups with $12.4 \lesssim \log M_{\rm halo} / M_{\odot} \lesssim 15.1$ (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 $Λ{\rm CDM}$ 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.

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