Estimating masses of dwarf spheroidal galaxies

Precise measurements of mass in dark matter dominated dwarf spheroidal galaxies are of great importance for testing the theories of structure formation. We use $N$-body simulations of the tidal evolution of a dwarf galaxy orbiting the Milky Way to generate mock kinematical data sets and use them to test the reliability of a simple mass estimator proposed by Wolf et al. The evolution of the initially disky dwarf galaxy embedded in a dark matter halo was traced for 10 Gyr on a rather tight orbit. After about half of the time a dwarf spheroidal galaxy is formed that retains some remnant rotation and a non-spherical shape. Observing the triaxial galaxy along each of its principal axes we measure its half-light radius and the line-of-sight velocity dispersion and use them to estimate the mass. We find that the mass is significantly overestimated when the dwarf is seen along the longest axis of the stellar component and underestimated when observed along the shortest axis. We provide a formula that quantifies the systematic error in the estimated mass with respect to the true one as a function of the galaxy shape and line of sight.