X-ray emission in early-type galaxies surveyed by ROSAT.

We have obtained an optically flux-limited sample of 34 elliptical and S0 galaxies, observed with high and low angular resolution detectors on the X-ray Rontgen Satellite (ROSAT). The survey is large enough to test the present picture of cooling flow models developed to explain the origin, behavior, and evolution of hot gas found to be present in early-type galaxies. For this sample of galaxies, we have established a consistent set of X-ray luminosities, and have determined X-ray gas temperatures for a subset of the survey. A steep relationship is found between the X-ray and optical luminosities with a slope of 2.72 $\pm$ 0.27, with a large dispersion about the correlation. We also find X-ray gas temperatures 2-3 times hotter than expected for some galaxies. The results are inconsistent with predictions of the standard cooling flow model for elliptical galaxies and we suggest a modification of the model that places environment in a central role in determining the X-ray emission properties. To effectively study properties of the interstellar gas in elliptical galaxies, it is critical to separate the two primary mechanisms for X-ray emission: (1) hot, interstellar gas and (2) integrated emission from distinct stellar-like X-ray sources. The increased spatial resolution of the ROSAT detectors over previous ones, in conjunction with longer exposure times, makes it theoretically possible to probe fainter X-ray galaxies (log$L\sb{\rm X}/L\sb{B} \sbsp{\sim}{<}$ 30.5 ergs$\sp{-1}/L\sb{\odot})$ for a determination of the X-ray emission mechanism. Here, we use radial surface brightness profiles to determine or set upper limits to the stellar contribution in the fainter X-ray galaxies of our sample. To fit the profiles, a beta model is used to describe the gaseous component and a de Vaucouleurs $r\sp{1/4}$ law is used to represent the stellar component. For many of the targets, we find that either model does well in tracing the data. Therefore, we cannot determine the primary emission mechanism in those galaxies. We discover that the gaseous component may still be significant in the remaining galaxies, as it is in X-ray bright elliptical galaxies, with an upper limit to the discrete source contribution of log$L\sb{\rm X}/L\sb{B}\ \sim$ 29.46 ergs$\sp{-1}/L\sb{\odot}.$PhDAstronomyPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/131394/2/9909860.pd