Mass-to-light ratios of groups and clusters of galaxies

We constrain the mass-to-light ratios, gas mass fractions, baryon mass fractions, and the ratios of total to luminous mass for a sample of eight nearby relaxed galaxy groups and clusters: A262, A426, A478, A1795, A2052, A2063, A2199, and MKW 4s. We use ASCA spatially resolved spectroscopic X-ray observations and ROSAT PSPC images to constrain the total and gas masses of these clusters. To measure cluster luminosities, we use galaxy catalogs resulting from the digitization and automated processing of the second generation Palomar Sky Survey plates calibrated with CCD images in the Gunn-Thuan g, r, and i bands. Under the assumption of hydrostatic equilibrium and spherical symmetry, we can measure the total masses of clusters from their intracluster gas temperature and density profiles. Spatially resolved ASCA spectra show that the gas temperature decreases with increasing distance from the center. By comparison, the assumption that the gas is isothermal results in an underestimate of the total mass at small radii and an overestimate at large cluster radii. We have obtained luminosity functions for all clusters in our sample. After correcting for background and foreground galaxies, we estimate the total cluster luminosity using Schechter function fits to the galaxy catalogs. In the three lowest redshift clusters where we can sample to fainter absolute magnitudes, we have detected a flattening of the luminosity function at intermediate magnitudes and a rise at the faint end. These clusters were fitted with a sum of two Schechter functions. The remaining clusters were well fitted with a single Schechter function. Assuming H-0 = 50 h(50) km s(-1) Mpc(-1), the measured mass-to-light ratios are similar to 100 h(50) M./L.. This, along with a high baryonic fraction, is indicative of a low-density universe with Omega0 similar to 0.15-0.2.