ABSTRACT. We present a new mass estimate for the Hercules dwarf spheroidal (dSph) galaxy, based on the revised velocity dispersion obtained by Adén et al. The removal of a significant foreground contamination using newly acquired Strömgren photometry has resulted in a reduced velocity dispersion. Using this new velocity dispersion of 3.72 ± 0.91 km s^-1, we find a mass of M ₃₀₀ = 1.9^+1.1 _–0.8 × 10⁶ M within the central 300 pc, which is also the half-light radius, and a mass of M ₄₃₃ = 3.7^+2.2 _–1.6 × 10⁶ M within the reach of our data to 433 pc, significantly lower than previous estimates. We derive an overall mass-to-light ratio of M ₄₃₃/L = 103⁺⁸³ _–48[M /L ]. Our mass estimate calls into question recent claims of a common mass scale for dSph galaxies. Additionally, we find tentative evidence for a velocity gradient in our kinematic data of 16 ± 3 km s^–1 kpc^–1, and evidence of an asymmetric extension in the light distribution at ~0.5 kpc. We explore the possibility that these features are due to tidal interactions with the Milky Way. We show that there is a self-consistent model in which Hercules has an assumed tidal radius of r_t = 485 pc, an orbital pericenter of r_p = 18.5 ± 5 kpc, and a mass within r_t of . Proper motions are required to test this model. Although we cannot exclude models in which Hercules contains no dark matter, we argue that Hercules is more likely to be a dark-matter-dominated system that is currently experiencing some tidal disturbance of its outer parts.

Key words: galaxies: dwarf; galaxies: formation; galaxies: fundamental parameters; galaxies: individual (Hercules); galaxies: kinematics and dynamics

Print publication: Issue 1 (2009 November 20)
Received 2009 June 26, accepted for publication 2009 October 6
Published 2009 November 4