Diffuse X-ray emission in three poor clusters of galaxies

We report on ROSAT PSPC soft X-ray observations of three poor clusters of galaxies at distances above 100 Mpc (cz > 8000 km s(-1)). In all three cases the emission is centered on the dominant member of the cluster, i.e., NGC 4104, NGC 6269, and NGC 6329, respectively. X-ray emission was detected out to radii of 400-600 kpc. The bolometric X-ray luminosities range from 2.6 to 8.6 x 10(42) ergs s(-1) The soft X-ray emission characteristics and the physical properties deduced from our observations of all three poor clusters resemble those of downscaled rich clusters. In each case, the soft X-ray spectrum is well represented by a thermal model with kT similar or equal to 1.1-1.3 keV and near-solar metallicity in the center, increasing to kT similar or equal to 1.4-1.6 keV toward the outer boundaries while the metallicity, Z, decreases to about 0.1 solar. Equally good fits can be achieved if the metallicity is left at the solar value and an additional gas component with kT similar or equal to 0.5 keV is introduced. The central electron densities in all three poor clusters studied here are enhanced with respect to a King profile by factors of 2-6. This, together with the results of the spectral fits, can be interpreted as either indicating the presence of cooling flows or of a two-phase medium in the central areas. The spatial electron density distribution in the outer regions of each cluster can be fitted by King profiles with core radii of 17-60 kpc and exponents of beta = 0.38-0.44. Using the derived radial temperature and density distributions, the total gravitating mass is obtained. We derive M-tot = 3.7 +/- 0.7 x 10(13) M. within a radius of 300 kpc for each of the three systems, as opposed to 10(14)-10(15) M. for rich clusters. We find that the L-X versus kT relation found by A. C. Edge and G. C. Stewart (1991) for rich clusters of galaxies scales into the domain of poor clusters and groups of galaxies. The spectral fits of the central regions show that none of the first-ranking galaxies of the three poor clusters hosts a Seyfert 1 active galactic nucleus that contributes significantly to the emission in the ROSAT band (0.1-2.4 keV).