Matter distribution in twelve X-ray clusters of galaxies

We have analyzed the physical properties of a sample of twelve clusters of galaxies observed in X-rays with the Einstein satellite Imaging Proportional Counter, with an improved method of analysis following our first analysis of two clusters (Gerbal et al. 1992, hereafter GDLL). The X-ray emitting gas is found to be close to isothermal at the spatial resolution of the IPC. We derive the profiles of the X-ray gas. The small core radii already found in GDLL are confirmed for this larger sample (R_X_~50-100 kpc), together with the small slopes (β~0.4) and large central densities ((3-30) 10^-3^ cm^-3^). Besides a β-model, we have also fitted the density distribution with a law having an exponential cutoff : only for a cluster (ABCG 665) is this law preferred, while for the other clusters the fits are equally good with both laws. With the assumption of hydrostatic equilibrium, we derive the binding matter mass and density profiles, and compare them to the other kinds of matter distributions. For three clusters (ABCG 1795, 1991 and 2142), using new optical data, we have also calculated the stellar matter distribution. The amount of binding matter is large, but for large enough radii the ICM contribution is important. The stellar component, negligible compared to the ICM, and a fortiori to the dark matter, appears more peaked than the X-ray gas, but proportional to the binding matter : light traces the mass. A strong over-density of dark matter relative to X-ray gas is found in the very central regions of most clusters. We show this result to be robust in view of the physical hypotheses used for its determination. The dark matter core radius is very small, in agreement with independent determinations recently made from the optical analysis of gravitational arcs. We propose various physical interpretations to these phenomena.