The flat density profiles of massive, and relaxed galaxy clusters

TL;DR

This study analyzes the density profiles of massive, relaxed galaxy clusters, finding that their dark matter profiles are flatter than -1 and that baryonic and dark matter distributions are correlated with cluster properties, supporting a two-phase formation scenario.

Contribution

It introduces a detailed analysis of density profiles in galaxy clusters using the SIM model, revealing correlations and supporting a two-phase formation process.

Findings

Dark matter density profiles are flatter than -1.

Total density profiles agree with dissipationless simulations.

Inner slope anti-correlates with BCG mass and effective radius.

Abstract

In the present paper, we studied by means of the SIM introduced in Del Popolo (2009), the total and DM density profiles, and the correlations among different quantities, observed by Newman et al. (2012a,b), in seven massive and relaxed clusters, namely MS2137, A963, A383, A611, A2537, A2667, A2390. Similarly to Newman et al. (2012a,b), the total density profile, in the radius range 0.003 - 0.03$r_{200}$, has a mean total density profile in agreement with dissipationless simulations. The slope of the DM profiles of all clusters is flatter than -1. The slope, $\alpha$, has a maximum value (including errors) of $\alpha=-0.88$ in the case of A2390, and minimum value $\alpha=-0.14$ for A2537. The baryonic component dominates the mass distribution at radii $< 5-10$ kpc, while the outer distribution is dark matter dominated. We found an anti-correlation among the slope $\alpha$, the effective radius, $R_e$, and the BCG mass, and a correlation among the core radius $r_{core}$, and $R_e$. Moreover, the mass in 100 kpc (mainly dark matter) is correlated with the mass inside 5 kpc (mainly baryons). The behavior of the total mass density profile, the DM density profile, and the quoted correlations can be understood in a double phase scenario. In the first dissipative phase the proto-BCG forms, and in the second dissipationless phase, dynamical friction between baryonic clumps (collapsing to the center) and the DM halo flattens the inner slope of the density profile. In simple terms, the large scatter in the inner slope from cluster to cluster, and the anti-correlation among the slope, $\alpha$ and $R_e$ is due to the fact that in order to have a total mass density profile which is NFW-like, clusters having more massive BCGs at their centers must contain less DM in their center. Consequently the inner profile has a flatter slope.