The Elongated Structure of the Hercules dSph from Deep LBT Imaging

Abstract

We present a deep, wide-field photometric survey of the newly-discovered Hercules dwarf spheroidal galaxy, based on data from the Large Binocular Telescope. Images in B, V and r were obtained with the Large Binocular Camera covering a 23' times 23' field of view to a magnitude of ~25.5 (5 sigma). This permitted the construction of colour-magnitude diagrams that reach approximately 1.5 magnitudes below the Hercules main sequence turnoff. Three-filter photometry allowed us to preferentially select probable Hercules member stars, and examine the structure of this system at a previously unattained level. We find that the Hercules dwarf is highly elongated (3:1), considerably more so than any other dSph satellite of the Milky Way except the disrupting Sagittarius dwarf. While we cannot rule out that the unusual structure is intrinsic to Hercules as an equilibrium system, our results suggest tidal disruption as a likely cause of this highly elliptical structure. Given the relatively large Galactocentric distance of this system (132 +/- 12 kpc), signs of tidal disruption would require the Hercules dwarf to be on a highly eccentric orbit around the Milky Way.