An optical X-ray study of Abell 576, a galaxy cluster with a cold core

We analyze the galaxy population and dynamics of the galaxy cluster A576; the observational constraints include 281 redshifts (230 new), R-band CCD galaxy photometry over a 2 h(-1) Mpc x 2 h(-1) Mpc region centered on the cluster, an Einstein IPC X-ray image, and an Einstein MPC X-ray spectrum. We focus on an 86% complete magnitude-limited sample (R(23.5) < 17) of 169 cluster galaxies. The cluster galaxies with emission lines in their spectra have a larger velocity dispersion and are significantly less clustered on this 2 h(-1) Mpc scale than galaxies without emission lines. We show that excluding the emission-line galaxies from the cluster sample decreases the velocity dispersion by 18% and the virial mass estimate by a factor of 2. The central cluster region contains a nonemission galaxy population and an intracluster medium which is significantly cooler (sigma(core) = 387(-105)(+250) km s(-1) and T-x = 1.6(-0.3)(+0.4) keV at 90% confidence) than the global populations (sigma = 977(-96)(+124) km s(-1) for the nonemission population and T-x > 4 keV at 90% confidence). Because (1) the low-dispersion galaxy population is no more luminous than the global population and (2) the evidence for a cooling flow is weak, we suggest that the core of A576 mag. 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 similar to 1 h(-1) Mpc is T-x similar to 8 keV (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 in A576 are typical of other clusters, the baryon fraction is relatively low. Using the consistent cluster binding mass, we show that the gas mass fraction is similar to 3 h(-3/2)% and the baryon fraction is similar to 4%.