Mass‐to‐Light Ratios of Groups and Clusters of Galaxies

Abstract

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 MKW4s. We useASCA 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 intra-cluster gas temperature and dens ity 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 ma ss 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 magni tudes and a rise at the faint end. These clusters were fitted with a sum of two Schechter functions. The remaining cl usters were well fitted with a single Schechter function. Assuming H0 = 50 h50 km s -1 Mpc -1 , the measured mass-to-light ratios are ∼ 100 h50 M⊙/L⊙. This, along with a high baryonic fraction, is indicative of a low density universe with 0 ∼ 0.15 - 0.2. Subject headings: Cosmology — galaxies: clusters: individual — intergalactic medium — X-rays: galaxies