A new method of estimating the mass-to-light ratio of the Ursa Minor dwarf spheroidal galaxy

摘要

Dwarf satellite galaxies undergo strong tidal forces produced by the maingalaxy potential. These forces disturb the satellite, producing asymmetries inits stellar distribution, tidal tail formation, and modifications of thevelocity dispersions profiles. Most of these features are observed in the UrsaMinor (UMi) dwarf spheroidal galaxy, which is one of the closest satellites ofthe Milky Way. These features show that UMi is been tidally disrupted andprobably not in virial equilibrium. The high velocity dispersion of UMi couldalso be a reflection of this tidal disruption and not the signature of thelarge dark matter content that would be deduced if virial equilibrium isassumed. In order to avoid the uncertainty produced when virial equilibrium isassumed in systems in strong tidal fields, we present a new method ofcalculating the mass-to-luminosity ratio of disrupted dwarf galaxies. Thismethod is based on numerical simulations and only takes into account the shapeof the dwarf density profile and the tidal tail brightness, but does not dependon the kinematics of the dwarf. Applying this method to UMi, we obtain amass-to-luminosity relation of 12, which is lower than the value obtainedassuming virial equilibrium (M/L=60). In addition, if UMi has a largedark-matter content, it will be impossible to reproduce a tidal tail asluminous as the one observed.