Shallow Dark Matter Cusps in Galaxy Clusters

TL;DR

This study uses high-resolution simulations to investigate how the initial compactness of progenitor galaxies influences the evolution of dark matter and stellar distributions in galaxy clusters, revealing that collisionless mergers can flatten dark matter cusps.

Contribution

It demonstrates that initial galaxy structure significantly affects the size-mass relation and dark matter cusp slopes in galaxy clusters, providing insights into observed shallow cusps.

Findings

More compact initial stellar distributions lead to steeper size-mass relations.

Collisionless mergers can reduce dark matter cusp slopes by 0.3 to 0.5.

Shallow dark matter cusps in clusters may originate from initial galaxy structures.

Abstract

We study the evolution of the stellar and dark matter components in a galaxy cluster of $10^{15} \, \rm{M_{\odot}}$ from $z=3$ to the present epoch using the high-resolution collisionless simulations of Ruszkowski & Springel (2009). At $z=3$ the dominant progenitor halos were populated with spherical model galaxies with and without accounting for adiabatic contraction. We apply a weighting scheme which allows us to change the relative amount of dark and stellar material assigned to each simulation particle in order to produce luminous properties which agree better with abundance matching arguments and observed bulge sizes at $z=3$. This permits the study of the effect of initial compactness on the evolution of the mass-size relation. We find that for more compact initial stellar distributions the size of the final Brightest Cluster Galaxy grows with mass according to $r\propto M^{2}$, whereas for more extended initial distributions, $r\propto M$. Our results show that collisionless mergers in a cosmological context can reduce the strength of inner dark matter cusps with changes in logarithmic slope of 0.3 to 0.5 at fixed radius. Shallow cusps such as those found recently in several strong lensing clusters thus do not necessarily conflict with CDM, but may rather reflect on the initial structure of the progenitor galaxies, which was shaped at high redshift by their formation process.