The cluster galaxy circular velocity function

We present galaxy circular velocity functions (GCVFs) for 34 low redshift (z < � 0.15) clusters identified in the Sloan Digital Sky Survey (SDSS), for fifteen clusters drawn from dark matter simulations of hierarchical structure growth in aCDM cosmology, and for �29,000 SDSS field galaxies. We find that the observed and simulated cluster GCVFs take the form of a power law. The cumulative GCVFs of the simulated clusters are very similar across a wide range of cluster masses, provided individual subhalo circular velocities are scaled by the circular velocities of the parent cluster. Once all sources of error are accounted for, the intrinsic scatter in the cumulative, scaled observed cluster GCVF is consistent with the simulations. The slope of the observed cluster GCVF is �-2.5, independent of cluster velocity dispersion. The average slope of the simulated GCVFs is somewhat steeper, although formally consistent given the errors. Using our highest resolution cluster, we find that the effects of baryons on galaxy rotation curves is to flatten the simulated cluster GCVF into better agreement with observations. Finally, we find that the field GCVF deviates significantly from a power law, being flatter than the cluster GCVF at circular velocities less than 200 kms 1 , and steeper at circular velocities greater than 200 kms 1 .