The Stellar Populations and Structural Properties of Ultra Faint Dwarf   Galaxies, Canes Venatici I, Bootes I, Canes Venatici II, and Leo IV
  We take deep images of four ultra faint dwarf (UFD) galaxies, Canes Venatici I (CVn I), Bootes I (Boo I), Canes Venatici II (CVn II), and Leo IV using the Suprime-Cam on the Subaru Telescope. The colour-magitude diagrams (CMDs) extend below the main sequence turn-offs (MSTOs) and yield measurements of the ages of stellar populations. The stellar populations of faint three galaxies, Boo I, CVn II and Leo IV dwarf spheroidal galaxies (dSphs) are estimated as old as a Galactic globular cluster M92. We confirm that Boo I dSph has no intrinsic colour spread in the MSTO, and no spatial difference in the CMD morphology, which indicates that Boo I dSph is composed of an old single stellar population. One of the brightest UFDs, CVn I dSph, shows a relatively younger age (-12.6 Gyr) with respect to Boo I, CVn II, and Leo IV dSphs, and the distribution of red horizontal branch (HB) stars is more concentrated toward the centre than that of blue HB stars, suggesting that the galaxy contains complex stellar populations. Boo I and CVn I dSphs show the elongated and distorted shapes. CVn II dSph has the smallest tidal radius as a Milky Way satellite and has distorted shape, while Leo IV dSph shows less concentrated spherical shape. The simple stellar population of faint UFDs indicates that the gases in their progenitors were removed more effectively than those of brighter dSphs at an occurrence of their initial star formation. This is reasonable if the progenitors of UFDs belong to the less massive halos than those of brighter dSphs. 
Sakurako Okamoto, Nobuo Arimoto, Yoshihiko Yamada, Masato Onodera
astro-ph.GA