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

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 nearly 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 v(r) = -107.7 +/- 1.0 kms(-1) and a velocity dispersion of sigma(nu) = 3.7(-1.1)(+1.2) kms(-1). With this updated velocity dispersion and a new literature measurement of the radial surface brightness profile, we derive a mass contained within the half-light radius of M (r < r(h)) = 4. 7(-2.9)(+3.0) x 10(6) M-circle dot. This mass corresponds to a mass-to-light ratio of [M/L]r(h) = 25(-16)(+17) M-circle dot/L-circle dot, demonstrating that And XXV is most-likely dark matter dominated. We also measure the metallicity of And XXV to be [Fe /H] = -1.9 +/- 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(DM) (150 pc) = 2.3(-1.1)(+1.4) x 10(7) M-circle dot kpc(-3), which makes And XXV a clear outlier when compared to other local group 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.

Automated summary

In this study, we find that And XXV is a dwarf spheroidal galaxy dominated by dark matter, with a relatively high mass and significant differences compared to other galaxies of similar mass.