The Hercules Satellite: A Stellar Stream in the Milky Way Halo?

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany ; Alexander von Humboldt research fellow
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

e-mail: shoko@ari.uni-heidelberg.de
e-mail: martin@mpia-hd.mpg.de

Abstract

We investigate the possibility that Hercules, a recently discovered Milky Way satellite, is a stellar stream in the process of formation. This hypothesis is motivated by Hercules’ highly elongated shape as well as the measurement of a tentative radial velocity gradient along its body (Adén et al. 2009a). The application of simple analytical techniques (Jin & Lynden-Bell 2007) on radial velocity data of its member stars provides tight constraints on the tangential velocity of the system (−16₋₂₂⁺⁶ kms⁻¹, relative to the Galactic Standard of Rest). Combining this with its large receding velocity (145 km s⁻¹) and distance (138 kpc) yields an orbit that would have taken Hercules to within 6₋₂⁺⁹ kpc of the Galactic centre approximately 0.6 Gyr ago. This very small perigalacticon can naturally explain the violent tidal destruction of the dwarf galaxy in the Milky Way’s gravitational potential, inducing its transformation into a stellar stream.