Andromeda XXV -- a dwarf galaxy with a low central dark matter density

Abstract

Andromeda (And) XXV has previously been reported as a dwarf spheroidal galaxy (dSph) with little-to-no dark matter. However, the uncertainties on this result were significant. In this study, we double the number of member stars and re-derive the kinematics and mass of And XXV. We find that And XXV has a systemic velocity of ${\nu }_{\mathrm{r}}=-107.7\pm 1.0{\mathrm{kms}}^{-1}$ and a velocity dispersion of ${\sigma }_{\nu }=4.5\pm 1.0{\mathrm{kms}}^{-1}$. With this better constrained velocity dispersion, we derive a mass contained within the half-light radius of $M(r<r_{\mathrm{h}})=6.9_{-2.8}^{+3.2}\times 10^6{\mathrm{M}}_{\odot }$. This mass corresponds to a mass-to-light ratio of ${[M/L]}_{{\mathrm{r}}_{\mathrm{h}}}=37_{-15}^{+17}{\mathrm{M}}_{\odot }/{\mathrm{L}}_{\odot }$, demonstrating, for the first time, that And XXV has an unambiguous dark matter component. We also measure the metallicity of And XXV to be $[Fe/H]=-1.9\pm 0.1$ dex, which is in agreement with previous results. Finally, we extend the analysis of And XXV to include mass modelling using GravSphere. We find that And XXV has a low central dark matter density, ${\rho }_{\mathrm{DM}}(150\mathrm{pc})=2.7_{-1.6}^{+1.8}\times 10^7{\mathrm{M}}_{\odot }{\mathrm{kpc}}^{-3}$, making And XXV a clear outlier when compared to other Local Group (LG) dSphs of the similar stellar mass. In a companion paper, we will explore whether some combination of dark matter cusp-core transformations and/or tides can explain And XXV's low density.

arXiv comment13 pages, 8 figures (7 main, 1 appendix). Submitted to MNRAS

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