Exploring Halo Substructure with Giant Stars XI

A new large-area Washington M,T2 + DDO51 filter survey of more than 10 deg2 around the Carina dSph galaxy reveals a spectroscopically confirmed power law radial density “break" population of Carina giant stars extending several degrees beyond the central King profile. Magellan telescope MIKE spectroscopy establishes the existence of Carina stars to at least 4.5 times its central King limiting radius, rlim and primarily along Carina’s major axis. To keep these stars bound to the dSph would require a global Carina mass-to-light ratio of M/L ≥ 6,300 (M/L)⊙. The MIKE velocities, supplemented with ∼ 950 additional Carina field velocities from archived VLT+GIRAFFE spectra with r . rlim, demonstrate a nearly constant Carina velocity dispersion (σv) to just beyond r = rlim, and both a rising σv and a velocity shear at still larger radii. Together, the observational evidence suggests that the discovered extended Carina population represents tidal debris from the dSph. Of 65 giant candidates at large angular radii from the Carina center for which MIKE spectra have been obtained 94% are associated either with Carina or a second, newly discovered diffuse, but strongly radial velocity-coherent (σv=9.8 km s−1), foreground halo system. The fifteen stars in this second, retrograde velocity population have (1) a mean metallicity ∼ 1 dex higher than that of Carina, and (2) colors and magnitudes consistent with the red clump of the Large Magellanic Cloud (LMC). Additional spectroscopy of giant star candidates in fields linking Carina and the LMC show a smooth velocity gradient between the LMC and the retrograde Carina moving group. We conclude that we have found Magellanic stars almost twice as far (22) from the LMC center than previously known. Subject headings: Carina Dwarf – galaxies: Local Group – kinematics and dynamics – Magellanic Clouds –cosmology : dark matter