A ROSAT HRI OBSERVATION OF THE ABELL-478 CLUSTER OF GALAXIES

We present an analysis of a high-resolution ROSAT image of the Abell 478 cluster of galaxies. The X-ray emission from the intracluster gas has been compared with the optical emission from the central cluster galaxy, and indicates a good agreement between the centroids, the ellipticities, and the orientations of the isophotes. We interpret this as indicating that the central galaxy is at rest with respect to the cluster potential. This is in apparent contradiction with the radial velocity measurements of Zabludoff, Huchra & Geller, which suggest that the central galaxy has a large peculiar velocity with respect to the other cluster galaxies. There is no evidence for substructure in the form of significant departures from smooth elliptical isophotes in the X-ray emission from the cluster, and we also find no significant deviations in the ROSAT X-ray image of Abell 2029, from which Sarazin, O'Connell & McNamara claim to detect filamentary emission. Finally, we determine the properties of the hot intracluster gas from deprojection of the X-ray images. The results are compared with those from an analysis of the Position Sensitive Proportional Counter image, and confirm the presence of a massive cooling flow of approximately 900 M. yr-1. The density profile also indicates that the Thomson depth in the cluster is 1 per cent within 70 kpc, and therefore that electron scattering of beamed optical continuum emission from obscured active nuclei could explain the extended blue light around distant radio galaxies. We also determine the. gas mass within 2.3 Mpc to be 3 x 10(14) M., indicating a large over-density of baryons with respect to the critical density of the Universe if standard primordial nucleosynthesis calculations are correct. This implies that there is much large-scale power in the Universe, or that OMEGA0 less than or similar to 0.3, or that there are problems with standard primordial nucleosynthesis.