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

Adén, D. and Wilkinson, M. I. and Read, J. I. and Feltzing, S. and Koch, Andreas and Gilmore, G. F. and Grebel, E.K. and Lundström, I. (2009) A new low mass for the Hercules dSph : the end of a common mass scale for the dwarfs? Astrophysical Journal Letters, 706 (1). L150-L154. ISSN 2041-8205

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 rt = 485 pc, an orbital pericenter of rp = 18.5 ± 5 kpc, and a mass within rt of $M_{{\rm tid},r_t}=5.2_{-2.7}^{+2.7} \times 10^6\,M_\odot$. 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.