ALBERT

Description: The impulse approximation and full N-body experiments are used to investigate the changes in mass and binding energy which result from gravitational encounters between spherical galaxies. Models with King (1966) and de Vaucouleurs (1948) surface density profiles, and radial, tangential, and isotropic velocity distributions. Results based on the impulse approximation depend on two collision parameters (the impact parameter and the ratio of the perturbing mass to the relative velocity at infinity); they agree well with N-body results. The asymptotic limit, valid for distant encounters, is found to approximate the correct results only for impact parameters larger than 10 effective radii. It is therefore of little use in most situations of interest. No simple 'linear' or 'quadratic' dependence of energy and mass loss on collision strength is found, but rather, a smooth intermediate behavior. Cross sections depend very little on the internal velocity structure of the galaxy, but quite strongly on the surface density profile in its outer parts. The reasons why these results disagree with the conclusions of previous workers are discussed.

Description: The approximately 86 ks ROSAT Position Sensitive Proportional Counter (PSPC) image of the Coma Cluster is deeper than any previous X-ray observation of a galaxy cluster. We search for X-ray emission from 35 individual galaxies in a magnitude-limited sample, all of which lie within 20 arcmins of the optical axis in at least one of the four Coma pointings. We detect seven galaxies in the 0.4-2.4 keV band at a significance level exceeding 3 sigma, and a further four at above 2 sigma. Although we can set only upper limits on the individual flux from each of the other galaxies, we are able to measure their mean flux by stacking the observations. The X-ray luminosities of the seven detections range from 6.2 x 10(exp 40) to 1.5 x 10(exp 42) ergs/s (0.4-2.4 keV for H(sub 0) = 50 km/s/Mpc). For galaxies with a blue absolute magnitude of about -21 we find a mean X-ray luminosity of 1.3 x 10(exp 40) ergs/s. The ratio of X-ray to optical luminosity is substantially smaller for such subjects than for the brightest galaxies in the cluster. The X-ray luminosities of the four brightest galaxies are ill-defined, however, because of ambiguity in distinguishing galaxy emission from cluster emission. Each object appears to be related to significant structure in the diffuse intracluster medium. We also investigate emission in the softer 0.2-0.4 keV band where detections are less significant because of the higher background, and we discuss the properties of a number of interesting individual sources. The X-ray luminosities of the Coma galaxies are similar to those of galaxies in the Virgo Cluster and in other regions with relatively low galaxy density. We conclude that large-scale environmental effects do not significantly enhance or suppress the average X-ray emission from galaxies, but that individual objects vary in luminosity substantially in a way which may depend on the detailed history of their environment.