1996A&A...305..435B

We present the results of a pointed ROSAT PSPC observation of the Abell cluster A 3558. A 3558 is the only cluster classified as richness class four in the entire ACO catalogue and, together with A 3556 and A 3562, it is part of an elongated structure defining the core of the Shapley Concentration. The X-ray surface brightness distribution of the cluster can be fitted as the sum of two components: an elliptical King law, with a major core radius of ∼0.20h^–1Mpc and an axial ratio ∼0.72, and a Gaussian central source associated with the cD galaxy. The centers of the two X-ray components are separated by 75 arcsec, corresponding to ∼50h^–1kpc. A number of additional discrete sources is also present in the field. Although most of them are probably unrelated to the cluster, two of these sources are identified with the poor clusters SC 1327-312 and SC 1329-313, at approximately the same redshift as A 3558, and two other sources are identified with bright galaxies, which are also part of the core of the Shapley Concentration. In addition, there is also evidence of an enhanced diffuse X-ray emission which connects the outer part of A 3558 with SC 1327-312. From the spectral analysis, we find that the temperature profile of A 3558 is consistent with an isothermal distribution at kT∼3.25keV, while the cD galaxy has the lower temperature kT=1.89keV. For the cluster we derive a luminosity L_X=1.1x10⁴⁴h^–2erg/s, in the energy range [0.5-2.0]keV, within a radius of 0.8h^–1Mpc and a total mass M_tot=3.1x10¹⁴h^–1M_☉ within an Abell radius (1.5h^–1Mpc). In the same region the gas mass is ∼20%h^–3/2 of the total mass and there are evidences for the hot gas to be less centrally concentrated than the dark matter and the stellar component. For h=1 most of the dynamical mass is in dark matter, while for h=0.5 most of the mass is in hot gas. In both cases, the stellar mass appears to be a good tracer of the total mass distribution. The derived M/L ratio is ∼130hM_☉/L_☉ for radii larger than ∼0.4h^–1Mpc. Finally, under the hypothesis that the central density spike represents a cooling flow, we derive a mass deposition rate of ∼25h^–2M_☉/yr. The single physical parameters derived for A 3558 show a general agreement with the typical values found for other X-ray emitting clusters, but the correlations between these quantities show some peculiarities, probably due to the complex dynamical state of this cluster.