THE BARYONIC FRACTION IN GROUPS OF GALAXIES FROM X-RAY MEASUREMENTS

The recent ROSAT X-ray detections of hot intergalactic gas in three gr oups of galaxies are reviewed, and the resulting baryonic fraction in these groups is reevaluated, We show that the baryonic fraction obtain ed, assuming hydrostatic equilibrium, should depend, perhaps sensitive ly, on the radius out to which the X-rays are detected, and the temper ature profile of the gas. We find that the NGC 2300 group has a baryon ic fraction out to 25' of at least 20%, thus over 5 times higher than in the original analysis of Mulchaey et al. and also much higher than one would obtain from big bang nucleosynthesis, but similar to the oth er two groups as well as rich clusters. With this baryonic fraction, g roups would be fair tracers of the distribution of baryons in the univ erse if Omega h(50)(2)=0.3. A baryonic fraction that increases with ra dius is consistent with the X-ray data from all three groups. However, a detailed analysis of the NGC 2300 group shows that the dependence o f baryonic fraction on radius is not well constrained by the data, in part because of uncertainties in the estimated background.