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 12 poor groups that have extensive membership information. Diffuse X-ray emission is detected in nine of these groups. In all but one of the X-ray-detected groups, the X-ray emission is centered on a luminous elliptical galaxy. Fits to the surface brightness profiles of the X-ray emission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy and is extended on scales of 20-40 h(-1) kpc. The location and extent of this component, combined with its X-ray temperature (similar to 0.7-0.9 keV) and luminosity (similar to 10(41-42) h(-2) ergs s(-1)), favor an origin in the interstellar medium of the central galaxy. Alternatively, the central component may be the result of a large-scale cooling flow. The second X-ray component is detected out to a radius of at least similar to 100-300 h(-1) kpc. This component follows the same relationships found among the X-ray temperature (T), X-ray luminosity (L-X), and optical velocity dispersion (sigma(r)) of rich clusters. This result suggests that the X-ray-detected groups are low-mass versions of clusters and that the extended gas component can properly be called the intragroup medium, by analogy to the intracluster medium in clusters. The failure to detect an intragroup medium in the three groups with very low velocity dispersions is consistent with their predicted X-ray luminosities and temperatures based on the relationships derived for clusters and X-ray-detected groups. The best-fit value of beta derived from the sigma(r)-T relationship for groups and clusters is similar to 0.99 +/- 0.08, implying that the galaxies and the hot gas trace the same potential with equal energy per unit mass and that the groups are dynamically relaxed. We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the position angle of the large-scale X-ray emission. This trend is consistent with that found for some rich clusters containing cD galaxies. The alignment of the central galaxy with the extended X-ray emission suggests that the formation and/or evolution of the central galaxy is linked to the shape of the global group potential. One possible scenario is that the central galaxy formed via galaxy-galaxy mergers early in the lifetime of the group and has not been subject to significant dynamical evolution recently.