Einstein observations of the Hydra A cluster and the efficiency of galaxy formation in groups and clusters

Abstract

We have analyzed the Einstein imaging proportional counter (IPC) and monitor proportional counter (MPC) observations of the poor cluster of galaxies centered on the radio galaxy Hydra A. The X-ray properties include diffuse emission with a characteristic temperature of 4.5 keV extending to at least 1 Mpc and an X-ray luminosity of 4 × 1044 ergs s-1 between 0.5-4.5 keV (for H0 = 50 km s-1 Mpc-1). We find that the cooling time of the gas in the core is short (∼5 × 109 yr). From the surface brightness profile, we infer that the X-ray-emitting gas in the Hydra A cluster must be condensing out of the intracluster medium at a rate of ≈600 M⊙ yr-1 (neglecting possible effects of heating and thermal conduction). This is one of the largest mass deposition rates observed in a cluster of galaxies. From the observed gas and galaxy distributions, we have computed the ratio of gas to galaxy mass and find that within 0.75 Mpc, the gas mass is a factor of 4h50-3/2 times that of the galaxies. We compare the ratio of gas mass to stellar mass for a variety of systems and find that this ratio correlates with the gas temperature. We explore the implications of this correlation on the efficiency of galaxy formation.