Baryon Fractions and Cluster Mass Estimates

Abstract

I report the current status on the determinations of the baryon fractions and dynamical/lensing masses of galaxy clusters as well as the $\sigma$ - $T$ relationship, making use of all the published data in literature which include 304 measurements for baryon fractions, 320/21 data points for masses of intracluster gas/cluster galaxies, (152+228)/72 data points for cluster masses derived from X-ray emitting gas/optical galaxies based on the hydrostatic equilibrium hypothesis, 46/33 measurements for the projected cluster masses obtained from strong/weak lensing. A straightforward statistical analysis reveals the following features: (1)The cluster baryon fraction indeed shows an increasing tendency towards a value of $\sim 30$ at large cluster radius; (2)The dynamical cluster masses are in fairly good agreement with the masses determined by weak lensing technique; (3)The cluster masses provided by strong lensing and other methods are essentially consistent within a factor of $\sim 2$. (4)The cluster $\sigma -T$ relationship is in concordance with the scenario that overall galaxy clusters are the well relaxed dynamical systems. These results are strongly suggestive of a low-mass density universe (${\Omega }_m<0.3$), in which the fraction of baryonic matter ($\sim 30$) is not very small !

8 pages + 4 PS figures, Talk given at the 5th Taipei Astrophysics Workshop: Cosmic Background Radiation and Large Scale Structures (PASP Conf. Ser.), Dec. 1997