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 twelve poor groups that have extensive membership information (Zabludoff & Mulchaey 1997; Paper I). 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 h100 −1 kpc. The location and extent of this component, combined with its X-ray temperature (∼ 0.7–0.9 keV) and luminosity (∼ 10 h100 −2 erg s), 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 ∼ 100–300 h100 −1 kpc. This component follows the same relationships found among the X-ray temperature (T), X-ray luminosity (LX) and optical velocity dispersion(σr) 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, in 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 β derived from the σr-T relationship for groups and clusters is ∼ 0.99±0.08, implying that the galaxies and hot gas trace the same potential with equal energy per unit mass and that the groups are dynamically relaxed.