The Properties of Poor Groups of Galaxies. II. X‐Ray and Optical Comparisons

We use ROSAT PSPC data to study the X-ray properties of a sample of twelvepoor groups that have extensive membership information (Zabludoff and Mulchaey1997; Paper I). Diffuse X-ray emission is detected in nine of these groups. Inall but one of the X-ray detected groups, the X-ray emission is centered on aluminous elliptical galaxy. Fits to the surface brightness profiles of theX-ray emission suggest the presence of two X-ray components in these groups.The first component is centered on the central elliptical galaxy. The locationand extent of this component, combined with its X-ray temperature andluminosity, favor an origin in the interstellar medium of the central galaxy.Alternatively, the central component may be the result of a large-scale coolingflow. The second X-ray component is detected out to a radius of at least100-300 kpc. This component follows the same relationships found among theX-ray temperature, X-ray luminosity and optical velocity dispersion of richclusters. This result suggests that the X-ray detected groups are low-massversions of clusters and that the extended gas component can properly be calledthe intragroup medium, in analogy to the intracluster medium in clusters. Wealso find a trend for the position angle of the optical light in the centralelliptical galaxy to align with the position angle of the large-scale X-rayemission. (Abridged)Comment: 38 pages, AASLaTeX with 16 PS figures. Figure 1a-1l available in gzipped postscript format at ftp://corvus.ociw.edu/pub/mulchae