A Massive Warm Baryonic Halo in the Coma Cluster

Several deep PSPC observations of the Coma cluster reveal a very large-scalehalo of soft X-ray emission, substantially in excess of the well knownradiation from the hot intra-cluster medium. The excess emission, previouslyreported in the central region of the cluster using lower-sensitivity EUVE andROSAT data, is now evident out to a radius of 2.6 Mpc, demonstrating that thesoft excess radiation from clusters is a phenomenon of cosmologicalsignificance. The X-ray spectrum at these large radii cannot be modelednon-thermally, but is consistent with the original scenario of thermal emissionfrom warm gas at ~ 10^6 K. The mass of the warm gas is on par with that of thehot X-ray emitting plasma, and significantly more massive if the warm gasresides in low-density filamentary structures. Thus the data lend vital supportto current theories of cosmic evolution, which predict that at low redshift\~30-40 % of the baryons reside in warm filaments converging at clusters ofgalaxies.Comment: Astrophysical Journal, in pres