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

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.

Analysis of the Density Profiles of Massive, Relaxed Galaxy Clusters

The paper by Newman et al. addresses the intricate task of dissecting the internal density profiles of massive, relaxed galaxy clusters into their stellar and dark matter (DM) components. By employing a multi-faceted approach that combines strong and weak gravitational lensing alongside stellar kinematics, the authors provide a detailed analysis for a sample of seven such clusters. The study leverages data spanning a vast radial range, thus enabling the decomposition of the total mass profiles into their luminous and dark constituents.

The key results point towards a mean inner DM profile that is significantly shallower than the canonical Navarro–Frenk–White (NFW) model, characterized by an average slope of $$\langle \beta \rangle = 0.50 \pm 0.10^{+0.14}_{-0.13}$$. Alternatively, cored NFW profiles with $\langle \log r_{\textrm{core}/\textrm{kpc}\rangle = 1.14 \pm 0.13^{+0.14}_{-0.22}$ are equally consistent with the observations. Notably, this demonstrates a significant departure from the steep inner cusps typically forecasted by DM-only simulations, especially within the central $\lesssim 30$ kpc, a region commensurate with the effective radii of the brightest cluster galaxies (BCGs).

A notable highlight of the study is the correlation between the inner DM profile and the stellar distribution within the BCGs. This implies a dynamic interaction between the formation and evolution of the BCGs and the DM halo. The authors convincingly argue that any comprehensive model of these clusters must consider the baryonic physics at play, including processes such as adiabatic contraction, galaxy mergers, and feedback from active galactic nuclei.

The research further delineates the stellar mass-to-light ratio from lensing and dynamic methodologies, finding consistency with stellar population synthesis models when adopting a Salpeter initial mass function. This consistency reinforces the study's methodology and its results on the DM profile.

Practical implications of this research include fostering refined theoretical models of galaxy formation and evolution that consider the dual influence of baryonic processes on DM halos. The analysis explicitly connects observed stellar structures to DM configurations, serving as a robust guide for future theoretical and simulation efforts.

On a theoretical level, the study poses significant implications for our understanding of DM halo structure. It points out that pure DM simulations may not fully capture the complex baryonic interactions shaping galaxy cluster cores. The evidence of a shallower DM profile in the presence of extensive baryonic mass suggests potential revisions to conventional theories that predominantly focus on cold dark matter (CDM) interactions without baryonic influence.

Looking towards future developments, this study lays the groundwork for refining DM interaction models, corroborating observational findings with increasingly sophisticated simulations, and potentially guiding observational strategies in both optical and upcoming radio surveys to further unravel the complex interplay between baryonic processes and dark matter in cluster-scale systems.