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 ell iptical galaxy. Fits to the surface brightness profiles of the X-ray e mission suggest the presence of two X-ray components in these groups. The first component is centered on the central elliptical galaxy and i s extended on scales of 20-40 h(-1) kpc. The location and extent of th is 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 flo w. 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 relation ships found among the X-ray temperature (T), X-ray luminosity (L-X), a nd optical velocity dispersion (sigma(r)) of rich clusters. This resul t suggests that the X-ray-detected groups are low-mass versions of clu sters and that the extended gas component can properly be called the i ntragroup medium, by analogy to the intracluster medium in clusters. T he failure to detect an intragroup medium in the three groups with ver y low velocity dispersions is consistent with their predicted X-ray lu minosities and temperatures based on the relationships derived for clu sters and X-ray-detected groups. The best-fit value of beta derived fr om 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 sam e potential with equal energy per unit mass and that the groups are dy namically relaxed. We also find a trend for the position angle of the optical light in the central elliptical galaxy to align with the posit ion angle of the large-scale X-ray emission. This trend is consistent with that found for some rich clusters containing cD galaxies. The ali gnment of the central galaxy with the extended X-ray emission suggests that the formation and/or evolution of the central galaxy is linked t o the shape of the global group potential. One possible scenario is th at the central galaxy formed via galaxy-galaxy mergers early in the li fetime of the group and has not been subject to significant dynamical evolution recently.