EINSTEIN-OBSERVATORY EVIDENCE FOR THE WIDESPREAD BARYON OVERDENSITY IN CLUSTERS OF GALAXIES

We analyse the X-ray surface brightness profiles of 19 moderately distant and luminous clusters of galaxies observed with the Einstein Observatory. Our aim is to determine cluster gas masses out to radii between 1 and 3 Mpc, and to confirm the apparent conflict, if Omega(0)=1, between the current calculations of the mean baryon fraction of the Universe expected from standard primordial nucleosynthesis, and the fraction of the mass in clusters which is in gas. Our analysis shows that baryon overdensities in clusters are much more widespread than just in the Coma cluster for which White & Frenk originally highlighted this problem. The uncertainties involved in our analysis and some cosmological implications from our results are briefly discussed. For a refined sample of 13 clusters we find that the baryon fraction for the gas within 1 Mpc lies between 10 and 22 per cent.