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

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  • D. Adén
  • M. I. Wilkinson
  • J. I. Read
  • S. Feltzing
  • Andreas Koch
  • G. F. Gilmore
  • E.K. Grebel
  • I. Lundström
<mark>Journal publication date</mark>4/11/2009
<mark>Journal</mark>Astrophysical Journal Letters
Issue number1
Number of pages5
Pages (from-to)L150-L154
Publication StatusPublished
<mark>Original language</mark>English


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.