An Optical/X--ray Study of A576, a Galaxy Cluster with a Cold Core

Abstract

We analyze the galaxy population and dynamics of the galaxy cluster Abell 576 using 281 redshifts (230 new), R band CCD galaxy photometry over a 2h^{-1} Mpc by 2h^{-1} Mpc region centered on the cluster, an Einstein IPC image, and an Einstein MPC spectrum. The cluster galaxies with emission lines in their spectra have a larger velocity dispersion and are significantly less clustered on this 2h^{-1} Mpc scale than galaxies without emission lines. We show that excluding the emission line galaxies decreases the velocity dispersion by 18% and the virial mass estimate by a factor of two. The central cluster region contains a non--emission galaxy population and an intracluster medium which are significantly cooler (sigma_{core}=387^{+250}_{-105}~km/s and T_X=1.6^{+0.4}_{-0.3}~keV at 90\% confidence) than the global populations (sigma=977^{+124}_{-96}~km/s for the non--emission population and T_X>4~keV at 90% confidence). Because (1) the low dispersion population is no more luminous than the global population and (2) the evidence for a cooling flow is weak, we suggest that the cluster core may contain the remnants of a lower mass subcluster. We examine the cluster mass, baryon fraction and luminosity function. The cluster virial mass varies significantly depending on the galaxy sample used. Consistency between the hydrostatic and virial estimators can be achieved if (1) the gas temperature at r\sim1h^{-1}~Mpc is T_X\sim8keV (the best fit value) and (2) several velocity outliers are excluded from the virial calculation. Although the best fit Schechter function parameters and the ratio of galaxy to gas mass are typical of other clusters, the baryon fraction is relatively low. Using a lower limit to the binding mass, we show that the gas mass fraction is \le3h^{-3/2}% and the baryon fraction is \le6%.