A new low mass for the Hercules dSph: The end of a common mass scale for the dwarfs?

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 M300 = 1.9+1.1-0.8 × 106 M⊙ within the central 300 pc, which is also the half-light radius, and a mass of M433 = 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 M433/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 rt = 485 pc, an orbital pericenter of r p = 18.5 5 kpc, and a mass within rt of Mtid,rt = 5.2+2.7-2.7 × 106 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. © 2009 The American Astronomical Society. All rights reserved.