DIFFUSE-X-RAY EMISSION FROM THE NGC-2300 GROUP OF GALAXIES - IMPLICATIONS FOR DARK MATTER AND GALAXY EVOLUTION IN SMALL-GROUPS

We report the detection of a hot, diffuse intragroup medium with the ROSAT Position Sensitive Proportional Counter in the NGC 2300 group of galaxies. This is the first time such a gas component has been found in a small group. The gas distribution is roughly symmetric, centered within approximately 3' of the elliptical NGC 2300, and extends to a radius of at least 0.2 h50(-1) Mpc. A Raymond-Smith (1977) hot plasma model provides an excellent fit to the X-ray spectrum, with a best-fit value temperature of 0.9(-0.14)+0.15 keV and abundance 0.06(-0.05)+0.12 solar. This temperature, combined with the assumption of gravitational confinement, leads to a total mass of the group of 3.0(-0.5)+0.4 x 10(13) M.. A reasonable estimate of the baryonic mass in galaxies and diffuse gas indicates that baryons can account for approximately 4% of this mass. A conspiracy of errors could push this number no higher than 10%-15%. This is one of the strongest pieces of evidence that dark matter dominates small groups such as this one, and is in agreement with dynamical arguments for masses of groups of galaxies. The intragroup medium in this system has the lowest metal abundance yet found in diffuse gas in a group or cluster. While not as dense as in clusters, the intragroup medium is still capable of significantly affecting the outer parts of galaxies in this group. In particular, the spiral galaxy NGC 2276 displays asymmetries in its optical and radio morphologies suggestive of an ongoing intragroup medium-galaxy encounter. This peculiar ''bowshock-shaped'' morphology is shared by spiral galaxies in other nearby groups, suggesting that it can be used as an optical indicator of the presence of a diffuse intragroup medium.