An X‐Ray and Optical Study of Matter Distribution in Clusters of Galaxies

Abstract

A sample of 12 Abell clusters of galaxies has been studied in the optical and X-ray bands. The optical data are derived from automatic photometry based on microdensitometric scans of Palomar Schmidt F-band plates. ROSAT PSPC data were obtained from the public archive. Galaxy density and X-ray luminosity profiles have been constructed and deprojected by parametric and nonparametric techniques to obtain volume density profiles. We find a relation between the gas density ρ_gas and the galaxy density ρ_gal consistent with the prediction of the hydrostatic isothermal model. We confirm that more than 70% of the cluster mass is dark within 1.5 Mpc h⁻¹₅₀ from the cluster center. Outside 250 kpc we find a nearly constant M/L_V ratio, which on average is 137 M_☉/L_☉, implying that the galaxy mass traces the total matter distribution. The dark mass-to-light ratio has a similar behavior. We also find a baryonic fraction f_b ≈ 0.2, which, assumed as representative of the cosmic value and compared with nucleosynthesis calculations, constrains the cosmological parameter Ω₀ to be smaller than 0.25.