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 A5 76; the observational constraints include 281 redshifts (230 new), R-b and CCD galaxy photometry over a 2 h(-1) Mpc x 2 h(-1) Mpc region cent ered 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 emiss ion lines in their spectra have a larger velocity dispersion and are s ignificantly less clustered on this 2 h(-1) Mpc scale than galaxies wi thout emission lines. We show that excluding the emission-line galaxie s 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 w hich is significantly cooler (sigma(core) = 387(-105)(+250) km s(-1) a nd T-x = 1.6(-0.3)(+0.4) keV at 90% confidence) than the global popula tions (sigma = 977(-96)(+124) km s(-1) for the nonemission population and T-x > 4 keV at 90% confidence). Because (1) the low-dispersion gal axy 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 A 576 mag. contain the remnants of a lower mass subcluster. We examine t he 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 achi eved if (1) the gas temperature at r similar to 1 h(-1) Mpc is T-x sim ilar to 8 keV (the best-fit value) and (2) several velocity outliers a re excluded from the virial calculation. Although the best-fit Schecht er function parameters and the ratio of galaxy to gas mass in A576 are typical of other clusters, the baryon fraction is relatively low. Usi ng the consistent cluster binding mass, we show that the gas mass frac tion is similar to 3 h(-3/2)% and the baryon fraction is similar to 4% .