SIMBAD references

As the largest gravitationally bound objects in the universe, clusters of galaxies may contain a fair sample of the baryonic mass fraction of the universe. Since the gas mass fraction from the hot intracluster medium is believed to be constant in time, the value of the cosmological deceleration parameter q₀ can be determined, in principle, by comparing the calculated gas mass fraction in nearby and distant clusters. To test the potential of this method, we compare the gas fractions derived for a sample of luminous (L_X>10⁴⁵ ergs.s^–1) nearby clusters with those calculated for eight luminous distant (0.3<z<0.6) clusters using ASCA and ROSAT observations. For consistency, we evaluate the gas mass fraction at a fixed physical radius of 1 h^–1₅₀ Mpc (assuming q₀=0.0). We find a best-fit value of q₀=0.07 with -0.47<q₀<0.67 at 95% confidence. This analysis includes both measurement errors and an intrinsic 25% scatter in the gas fractions due to the effects of cooling flows and mergers. We also determine the gas fraction using the method of Evrard, Metzler, and Navarro to find the total mass within r₅₀₀, the radius where the mean overdensity of matter is 500 times the critical density. In simulations, this method reduces the scatter in the determination of gravitational mass without biasing the mean. We find that it also reduces the scatter in actual observations for nearby clusters but not as much as simulations suggest. Using this method, the best-fit value is q₀=0.04 with -0.50<q₀<0.64. The excellent agreement between these two methods suggests that this may be a useful technique for determining q₀. The constraints on q₀ should improve as more distant clusters are studied and precise temperature profiles are measured to large radii.