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 Aden et al. The removal of a significant foreground contamination using newly acquired Stromgren 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(-0.8)(+1.1) x 10(6) M(circle dot) within the central 300 pc, which is also the half-light radius, and a mass of M(433) = 3.7(-1.6)(+2.2) x 10(6) M(circle dot) 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(-48)(+83)[M(circle dot)/L(circle dot)]. 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 similar to 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,rt) = 5.2(-2.7)(+2.7) x 10(6) M(circle dot). 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.