Einstein Observatory evidence for the widespread baryon overdensity in clusters of galaxies

Abstract

We analyse the X-ray surface brightness profiles of 19 moderatelydistant and luminous clusters of galaxies observed with the EinsteinObservatory. Our aim is to determine cluster gas masses out to radiibetween 1 and 3 Mpc, and to confirm the apparent conflict, if Omega_0=1,between the current calculations of the mean baryon fraction of theUniverse expected from standard primordial nucleosynthesis, and thefraction of the mass in clusters which is in gas. Our analysis showsthat baryon overdensities in clusters are much more widespread than justin the Coma cluster for which White & Frenk originally highlightedthis problem. The uncertainties involved in our analysis and somecosmological implications from our results are briefly discussed. For arefined sample of 13 clusters we find that the baryon fraction for thegas within 1 Mpc lies between 10 and 22 per cent.