Mass modelling the Andromeda Dwarf Galaxies: Andromeda VI and Andromeda XXIII

ABSTRACT Accurately mapping the mass profiles of low mass dwarf spheroidal (dSph) galaxies allows us to test predictions made by dark matter (DM) models. To date, such analyses have primarily been performed on Milky Way (MW) satellites. Meanwhile, the Andromeda galaxy (M31) is home to 35 known dwarf galaxies, yet only two have been successfully mass-modelled so far. A more comprehensive study of Local Group dwarfs is necessary to better understand the nature of dark matter. In this study, we have undertaken a dynamical study of two higher-luminosity Andromeda dwarf galaxies: Andromeda VI (And VI) and Andromeda XXIII (And XXIII). We infer an enclosed mass for And VI of M(r $<$ r$_{h}$) = (4.9 $\pm$ 1.5) $\times$ 10$^{7}$ M$_{\odot }$, corresponding to a mass-to-light ratio of $[M/L]_{r_{\rm {h}}}$ = (27.1 $\pm$ 8.2) M$_{\odot }$ L$_{\odot }$−1. We infer an enclosed mass for And XXIII of M(r $<$ r$_{h}$) = (3.1 $\pm$ 1.9) $\times$ 10$^{7}$ M$_{\odot }$, corresponding to a mass-to-light ratio of $[M/L]_{r_{\rm {h}}}$ = (90.2 $\pm$ 53.9) M$_{\odot }$ L$_{\odot }$−1. Using the dynamical Jeans modelling tool, gravsphere, we determine And VI and And XXIII’s dark matter density at 150 pc, finding $\rho _{\rm {DM,VI}}$ (150 pc) = (1.4 $\pm$ 0.5) $\times 10^{8}$ M$_{\odot }$ kpc$^{-3}$ and $\rho _{\rm {DM,XXIII}}$ (150 pc) = 0.5$\substack{+0.4 \\ -0.3} \times 10^{8}$ M$_{\odot }$ kpc$^{-3}$. Our results make And VI the first mass-modelled M31 satellite to fall into the cuspy regime. And XXIII has a lower density, implying either a more cored central dark matter density, or a lowering of the density through tides, with early quenching times disfavoring core formation via stellar feedback. This adds And XXIII to a growing list of M31 dwarfs with a central density lower than most MW dwarfs and lower than expected for isolated dwarfs in the Standard Cosmology. This could be explained by the M31 dwarfs having experienced stronger tides than their MW counterparts.