Investigating the dark matter halo of NGC 5128 using a discrete dynamical model

Abstract

Context. As the nearest accessible massive early-type galaxy, NGC 5128 presents an exceptional opportunity to measure dark matter halo parameters for a representative elliptical galaxy. Aims. Here we take advantage of rich new observational datasets of large-radius tracers to perform dynamical modeling of NGC 5128 Methods. We used a discrete axisymmetric anisotropic Jeans approach with a total tracer population of nearly 1800 planetary nebulae, globular clusters, and dwarf satellite galaxies extending to a projected distance of ∼250 kpc from the galaxy center to model the dynamics of NGC 5128. Results. We find that a standard Navarro-Frenk-White (NFW) halo provides an excellent fit to nearly all the data, except for a subset of the planetary nebulae that appear to be out of virial equilibrium. The best-fit dark matter halo has a virial mass of Mvir = 4.4−1.4+2.4 × 1012 M⊙, and NGC 5128 appears to sit below the mean stellar mass–halo mass and globular cluster mass–halo mass relations, which both predict a halo virial mass closer to Mvir ∼ 1013 M⊙. The inferred NFW virial concentration is cvir = 5.6−1.6+2.4, which is nominally lower than cvir ∼ 9 predicted from published cvir–Mvir relations, but within the ∼30% scatter found in simulations. The best-fit dark matter halo constitutes only ∼10% of the total mass at one effective radius but ∼50% at five effective radii. The derived halo parameters are consistent within the uncertainties for models with differing tracer populations, anisotropies, and inclinations. Conclusions. Our analysis highlights the value of comprehensive dynamical modeling of nearby galaxies and the importance of using multiple tracers to allow cross-checks for model robustness.