A New Low Mass for the Hercules dSph: The End of a Common Mass Scale for the Dwarfs?

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 _sun 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 _sun 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 _sun/L _sun]. 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 M_{tid,r_t}=5.2_{-2.7}^{+2.7} × 10^6 M_⊙. 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.