The Density Profiles of Massive, Relaxed Galaxy Clusters. II. Separating Luminous and Dark Matter in Cluster Cores

TL;DR

This study decomposes the mass profiles of massive galaxy clusters into stellar and dark matter components, revealing shallower dark matter profiles than standard models and exploring their connection to galaxy formation processes.

Contribution

It provides detailed measurements of dark matter density slopes and core sizes in galaxy clusters, and compares these to theoretical models of baryon-DM interactions.

Findings

Inner dark matter slopes are shallower than NFW predictions.

Cored NFW profiles fit the data as well as power-law models.

Inner dark matter profiles correlate with stellar distributions.

Abstract

We present stellar and dark matter (DM) density profiles for a sample of seven massive, relaxed galaxy clusters derived from strong and weak gravitational lensing and resolved stellar kinematic observations within the centrally-located brightest cluster galaxies (BCGs). In Paper I of the series, we demonstrated that the total density profile derived from these data, which span 3 decades in radius, is consistent with numerical DM-only simulations at radii >~ 5-10 kpc, despite the significant contribution of stellar material in the core. Here we decompose the inner mass profiles of these clusters into stellar and dark components. Parametrizing the DM density profile as a power law rho_DM ~ r^{-\beta} on small scales, we find a mean slope <\beta> = 0.50 +- 0.10 (random) +0.14-0.13 (systematic). Alternatively, cored Navarro-Frenk-White (NFW) profiles with <log r_core/kpc> = 1.14 +- 0.13 (random) +0.14-0.22 (systematic) provide an equally good description. These density profiles are significantly shallower than canonical NFW models at radii <~ 30 kpc, comparable to the effective radii of the BCGs. The inner DM profile is correlated with the distribution of stars in the BCG, suggesting a connection between the inner halo and the assembly of stars in the central galaxy. The stellar mass-to-light ratio inferred from lensing and stellar dynamics is consistent with that inferred using stellar population synthesis models if a Salpeter initial mass function is adopted. We compare these results to theories describing the interaction between baryons and DM in cluster cores, including adiabatic contraction models and the possible effects of galaxy mergers and active galactic nucleus feedback, and evaluate possible signatures of alternative DM candidates.