Dark and luminous mass components of Omega Centauri with stellar kinematics

Abstract

We combine proper motion data from $Gaia$ EDR3 and HST with line-of-sight velocity data to study the stellar kinematics of the Omega Centauri globular cluster. Using a steady-state, axisymmetric dynamical model, we measure the distribution of both the dark and luminous mass components. Assuming both Gaussian and NFW mass profiles, depending on the dataset, we measure an integrated mass of $\lesssim 10^6$ M${}_{\odot }$ within the Omega Centauri half-light radius for a dark component that is distinct from the luminous stellar component. For the HST and radial velocity data, models with a non-luminous mass component are strongly statistically preferred relative to a stellar mass-only model with a constant mass-to-light ratio. While a compact core of stellar remnants may account for a dynamical mass up to $\sim 5\times 10^5$ M${}_{\odot }$, they likely cannot explain the higher end of the range. This leaves open the possibility that this non-luminous dynamical mass component is comprised of non-baryonic dark matter. In comparison to the dark matter distributions around dwarf spheroidal galaxies, the Omega Centauri dark mass component is much more centrally concentrated. Interpreting the non-luminous mass distribution as particle dark matter, we use these results to obtain the J-factor, which sets the sensitivity to the annihilation cross section. For the datasets considered, the range of median J-factors is $\sim 10^{22}-10^{24}$ GeV${}^2$ cm${}^{-5}$, which is larger than that obtained for any dwarf spheroidal galaxy.

arXiv comment14 pages with 8 figures. Accepted for publication in MNRAS

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