Two-phase intracluster medium in the Centaurus Cluster of galaxies

ASCA and ROSAT data of the Centaurus Cluster, containing a cD galaxy NGC 4696, were analyzed in a partial reanalysis of the work of Fukazawa et al., Fabian et al., and Alien & Fabian. Radial brightness profiles in different energy bands show that the central excess emission of this cluster, seen previously in soft X-rays (<4 keV), is also present in the hard energy band up to 10 keV. Therefore, the central excess emission cannot be explained solely by a temperature drop together with a King-type potential with a flat core, and requires a deeper central potential. A double-beta brightness distribution gives a good account of the data. A deprojected energy spectrum within a spherical region of radius similar to 30 kpc at the cluster center indicates that the ICM cannot be isothermal there. Simultaneous fitting of the three (PSPC, GIS, and SIS) energy spectra extracted from the central region within a projected radius of 3' showed that a two-temperature model with temperatures of 1.4 and 4.4 keV and a common metallicity of 1.0 solar, with the Galactic absorption, gives the best description of the spectra. A cooling-flow spectral model did not give a satisfactory fit. These results reconfirm the two-phase picture developed by Fukazawa et al., in which the hot ICM fills nearly the entire cluster volume, where a small amount of cooler plasma is intermixed and localized near the cluster center. A three-dimensional cluster model incorporating the two-phase picture, the double-beta brightness distribution, and the central metallicity increase reproduced the overall ASCA and ROSAT data successfully. The spatial distribution of the dark matter that is derived by subtracting the stellar mass from the calculated total gravitating mass deviates from a King-type profile and exhibits a central excess. Another two-phase cluster model in which the dark matter density profile is given with the universal density profile of Navarro, Frenk, & White also gave a satisfactory account of the data. A detailed comparison of the iron mass distribution with that of the stellar component reveals that the iron is more widely spread than stars, perhaps as a result of energetic supernovae and the motion of the cD galaxy. Since the derived characteristics of the cool phase, including the temperature, angular extent, gas mass, and metallicity, are on a smooth extension of those of the interstellar medium (ISM) of elliptical galaxies, the cool phase can be regarded as the ISM associated with the cD galaxy, while the high X-ray luminosity of the cool phase (1.1 x 10(43) ergs s(-1) in 0.5-4.0 keV) is interpreted as a result of compression by the surrounding hot phase. The cool-phase X-ray emission is presumably sustained by energies dissipated by mass infalling to the cD galaxy in the course of cluster evolution.