The Dark Matter Density Profile in Brightest Cluster Galaxies

The inner dark matter (DM) density slope is an important test of our understanding of DM and its interplay with baryons during galaxy formation. Two of the main challenges in comparing to numerical simulations are measuring the mass profile over a wide radial range, and separating the contributions of the baryons and DM. I will show that the cores of galaxy clusters are an opportune place to make progress on these fronts due to the several precise observational tools that are available. By combining strong and weak gravitational lensing with spatially-resolved stellar kinematic measures within the centrally-located brightest cluster galaxies (BCGs), we have measured the total and DM density profiles in 7 relaxed galaxy clusters from radii of ~3 kpc to 3 Mpc. Surprisingly, we found that the slope of the total density profile follows that seen in CDM-only simulations quite closely, even at small radii where the contribution of stellar mass in the BCG is significant. Correspondingly, when the DM is isolated, its inner density slope is systematically shallower than a CDM-type cusp. Furthermore, the inner DM slope appears to correlate with properties of the BCG, providing a clue that DM in the cluster core is affected by the assembly history of the central galaxy. I will briefly discuss several physical scenarios that might explain these findings and evaluate their likelihood in light of our observations