Moderate Steepening of Galaxy Cluster Dark Matter Profiles by Baryonic Pinching

TL;DR

This study uses cosmological simulations to evaluate how baryonic processes, like star formation and cooling flows, influence the steepening of dark matter profiles in galaxy clusters, finding moderate effects after corrections.

Contribution

It provides the first detailed quantification of baryonic pinching effects on dark matter profiles in galaxy clusters, including corrections for cooling flows and gravitational adjustments.

Findings

Baryonic pinching causes a steepening of dark matter profiles by Δα~0.5-0.6 before corrections.

After correcting for cooling flows, the steepening reduces to Δα~0.1.

Total matter density profiles are about 0.5 steeper than dark matter-only profiles.

Abstract

To assess the effect of baryonic ``pinching'' of galaxy cluster dark matter (DM) haloes, cosmological (LCDM) TreeSPH simulations of the formation and evolution of two galaxy clusters have been performed, with and without baryons included. The simulations with baryons invoke star formation, chemical evolution with non-instantaneous recycling, metallicity dependent radiative cooling, strong star-burst, driven galactic super-winds and the effects of a meta-galactic UV field, including simplified radiative transfer. The two clusters have T_X~3 and 6 keV, respectively, and, at z~0, both host a prominent, central cD galaxy. Comparing the simulations without and with baryons, it is found for the latter that the inner DM density profiles, r<50-100 kpc, steepen considerably: Delta(alpha)~0.5-0.6, where -alpha is the logarithmic DM density gradient. This is mainly due to the central stellar cDs becoming very massive, as a consequence of the onset of late time cooling flows and related star formation. Once these spurious cooling flows have been corrected for, and the cluster gravitational potentials dynamically adjusted, much smaller pinching effects are found: Delta(alpha)~0.1. Including the effects of baryonic pinching, central slopes of alpha~1.0 and 1.2 are found for the DM in the two clusters, interestingly close to recent observational findings. For the simulations with baryons, the inner density profile of DM+ICM gas combined is found to be only very marginally steeper than that of the DM, Delta(alpha)<0.05. However, the total matter inner density profiles are found to be Delta(alpha)~0.5 steeper than the inner profiles in the dark matter only simulations.