The SLUGGS Survey: The Inner Dark Matter Density Slope of the Massive Elliptical Galaxy NGC 1407

TL;DR

This study models the dark matter profile of galaxy NGC 1407 using multiple tracers and Bayesian methods, revealing a central cusp and orbit characteristics that challenge standard halo contraction models.

Contribution

It introduces a hierarchical Bayesian approach with multiple kinematic tracers and stellar population data to accurately determine the dark matter density slope in a massive elliptical galaxy.

Findings

Evidence for a central cusp with a slope of approximately 1.0.

Support for suppressed halo steepening in massive galaxies.

Distinct orbital biases for different globular cluster populations.

Abstract

We investigate the dark matter density profile of the massive elliptical galaxy, NGC 1407, by constructing spherically symmetric Jeans models of its field star and globular cluster systems. Two major challenges in such models are the degeneracy between the stellar mass and the dark matter halo profiles, and the degeneracy between the orbital anisotropy of the tracer population and the total mass causing the observed motions. We address the first issue by using new measurements of the mass-to-light ratio profile from stellar population constraints that include a radially varying initial mass function. To deal with the mass--anisotropy degeneracy, we make use of multiple kinematic tracers, including two subpopulations of globular clusters in addition to the galaxy's field stars. We create a hierarchical Bayesian model that addresses several often neglected systematic uncertainties, such as the statistical weight given to various datasets and the adopted distance. After sampling the posterior probability distribution with a Markov Chain Monte Carlo method, we find evidence for a central cusp with a log-slope of $\gamma = 1.0^{+0.3}_{-0.4}$. This is lower than expected for dark matter halos that have undergone adiabatic contraction, supporting inferences from gravitational lensing that some process has suppressed the steepening of halos in massive galaxies. We also confirm radially-biased orbits for the metal-rich globular clusters and tangentially-biased orbits for the metal-poor globular clusters, which remains a puzzling finding for an accretion-dominated halo.