Astronomy and Astrophysics – Astrophysics
Scientific paper
1993-12-08
Astrophys.J. 421 (1994) L63
Astronomy and Astrophysics
Astrophysics
ApJ Lett 421 (Feb 1 1994), in press. 4 pages of uuencoded compressed Postscript (extract on UNIX with 'csh' after removing hea
Scientific paper
10.1086/187188
The recent {\sl ROSAT \/} X-ray detections of hot intergalactic gas in three groups of galaxies are reviewed and the resulting baryonic fraction in these groups is reevaluated. We show that the baryonic fraction obtained, assuming hydrostatic equilibrium, should depend, perhaps sensitively, on the radius out to which the X-rays are detected, and the temperature profile of the gas. We find that the NGC 2300 group has a baryonic fraction out to $25'$ of at least 20\%, thus over five times higher than in the original analysis of Mulchaey \etal\ (1993), and also much higher than one would obtain from big-bang nucleosynthesis, but similar to the other two groups as well as rich clusters. With this baryonic fraction, groups would be fair tracers of the distribution of baryons in the Universe if $\Omega h_{50}^2 = 0.3$. A baryonic fraction that increases with radius is consistent with the X-ray data from all three groups. However, a detailed analysis of the NGC 2300 group shows that the dependence of baryonic fraction on radius is not well constrained by the data, in part because of uncertainties in the estimated background.
Henriksen Mark J.
Mamon Gary A.
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