COSMOLOGICAL IMPLICATIONS OF ROSAT OBSERVATIONS OF GROUPS AND CLUSTERS OF GALAXIES

We have combined ROSAT PSPC and optical observations of a sample of gr oups and clusters of galaxies to determine the fundamental parameters of these systems (e.g., the dark matter distribution, gas mass fractio n, baryon mass fraction, mass-to-light ratio, and the ratio of total-t o-luminous mass). Imaging X-ray spectroscopy of groups and clusters sh ow that the gas is essentially isothermal beyond the central region, i ndicating that the total mass density (mostly dark matter) scales as r ho(dark) proportional to r(-2). The density profile of the hot X-ray-e mitting gas is fairly flat in groups with rho(gas) proportional to r(- 1.0) and becomes progressively steeper in hotter richer systems, with rho(gas) proportional to r(-2.0) in the richest clusters. These result s show, that in general, the hot X-ray-emitting gas is the most extend ed mass component in groups and clusters, the galaxies are the most ce ntrally concentrated component, and the dark matter is intermediate be tween the two. The flatter density profile of the hot gas compared to the dark matter produces a gas mass fraction that increases with radiu s within each object. There is also a clear trend of increasing gas ma ss fraction (from 2% to 30%) between elliptical galaxies and rich clus ters due to the greater detectable extent of the X-ray emission in ric her systems. For the few systems in which the X-ray emission can be tr aced to the viriaI radius (where the overdensity delta approximate to 200), the gas mass fraction (essentially the baryon mass fraction) app roaches a roughly constant value of 30%, suggesting that this is the t rue primordial value. Based on standard big bang nucleosynthesis, the large baryon mass fraction implies that Omega = 0.1-0.2. The antibiase d gas distribution suggests that feedback from galaxy formation and hy drodynamics play important roles in the formation of structure on the scale of galaxies to rich clusters. All the groups and clusters in our sample have mass-to-light ratios of M/L(v) similar to 100-150 M./L., which strongly contrasts with the traditional view that the mass-to-li ght ratio of rich clusters is significantly greater than individual ga laxies or groups with M/L(v) similar to 250-300 M./L.. We also show th at M/L(v) is essentially constant within the virial radius of clusters (where delta greater than or similar to 200), which is consistent wit h the peaks formalism of biased galaxy formation. While the mass-to-li ght ratios of groups and clusters are comparable (indicating a constan t mass fraction of optically luminous material), the ratio of the tota l mass-to-luminous mass (gas plus stars) monotonically decreases betwe en galaxies and clusters. The decrease in M(tot)/M(lum) arises from tw o factors: (1) the composition of baryonic matter varies from a predom inance of optically luminous material (stars) on the scale of galaxies (similar to 10 kpc) to a predominance of X-ray luminous material (hot gas) on the scale of rich clusters (similar to 1 Mpc), and (2) the ho t gas has a more extended spatial distribution than the gravitating ma tter. The observed decrease in M(tot)/M(lum) between galaxies and clus ters indicates that the universe actually becomes ''brighter'' on mass scales between 10(12) and 10(15) M., in the sense that a greater frac tion of the gravitating mass is observable.