Andromeda XXV -- a dwarf galaxy with a low central dark matter density
Résumé
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 νr=−107.7±1.0 kms−1 and a velocity dispersion of σν=4.5±1.0 kms−1. With this better constrained velocity dispersion, we derive a mass contained within the half-light radius of M(r<rh)=6.9+3.2−2.8×106 M⊙. This mass corresponds to a mass-to-light ratio of [M/L]rh=37+17−15 M⊙/L⊙, 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±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, ρDM(150pc)=2.7+1.8−1.6×107 M⊙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.
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