Weighing dark matter in brightest cluster galaxies

Abstract

Despite being a fundamental property that tests galaxy formation models, the measurement of elliptical galaxy masses is notoriously difficult. However, constraining the mass profiles of galaxy clusters is more tractable as these objects present us with a number of independent mass probes, such as lensing, X-ray, and stellar kinematic measurements. Lensing and X-ray analyses can provide stringent mass constraints up to the virial radius, but become unfeasible within _ 50 kpc due to the instrumental resolution and/or substructure in the central parts of clusters. On the other hand, complementary stellar kinematics of the central, gas-poor brightest cluster galaxy (BCG) can extend over the range _ 1–200 kpc in favourable circumstances. The fact that BCGs lie at the bottom of the potential well of non-interacting clusters, means that the stellar dynamics are indispensable in disentangling dark matter (DM) and stellar mass of galaxy clusters on relatively small scales. In this study, spatially-resolved stellar kinematics and WFPC2/F814W (I-band) Hubble images were used to study the stellar and dark matter distributions of a sample of 18 nearby BCGs. Furthermore, these mass distributions were analysed within one effective radius (1 ae), which denotes the radius in which half the total light of a galaxy is contained. This study employed the Multi Gaussian Expansion (MGE) formalism to accurately parametrise the BCG stellar mass from the original F814W images, as well as the DM profiles that were parametrised by the generalised-Navarro-Frenk-White (gNFW) formula. Secondly, a non-parametric, smoothing algorithm, known as LOESS, was employed in tandem with 3000 Monte-Carlo simulations to smooth the velocity dispersion profiles later incorporated in the Jeans analysis. Thirdly, this study iterated the Jeans Anisotropic MGE (JAM) routine—which implements the solution of the anisotropic Jeans equations—over a large, five-dimensional free-parameter space in an attempt to reproduce the BCG dispersion profiles, and thus acquire constraints on physical quantities through a minimum _2 statistical analysis. Furthermore, _2 maps were constructed to assist in understanding the structure and dependencies in the free-parameter space. An important novelty of this study is that the anisotropy parameter, _—parametrising orbital properties of a galaxy—was left as a free parameter, and therefore not a priori assumed to be isotropic as is traditionally done in many other studies. Results from this study indicated that the best-fit, mean stellar mass-to-light ratio in the F814W filter evaluates to h_?;I i _ 4 __;I for the sample of BCGs. The best-fit _ solutions showed a strong tendency towards mildly, radially-biased solutions (_ _ 0:2). On the other hand, the best-fit DM inner slope (_) solutions exhibited a bimodal distribution in which _ 44% of BCGs preferred shallow slopes (_ < 1), and _ 56% preferred cuspy slopes (_ > 1). On average for this BCG sample, stellar mass represent _ 73% of the total mass budget within 1 ae, whereas DM takes up _ 27%. Multiwavelength approaches, longer stellar kinematic profiles, more generalised dynamical models, and larger BCG samples will promote progress in disentangling the stellar mass and DM distributions of clusters in future studies