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We take deep images of four ultra faint dwarf (UFD) galaxies, Canes VenaticiI (CVn I), Bootes I (Boo I), Canes Venatici II (CVn II), and Leo IV using theSuprime-Cam on the Subaru Telescope. The colour-magitude diagrams (CMDs) extendbelow the main sequence turn-offs (MSTOs) and yield measurements of the ages ofstellar populations. The stellar populations of faint three galaxies, Boo I,CVn II and Leo IV dwarf spheroidal galaxies (dSphs) are estimated as old as aGalactic globular cluster M92. We confirm that Boo I dSph has no intrinsiccolour spread in the MSTO, and no spatial difference in the CMD morphology,which indicates that Boo I dSph is composed of an old single stellarpopulation. One of the brightest UFDs, CVn I dSph, shows a relatively youngerage (-12.6 Gyr) with respect to Boo I, CVn II, and Leo IV dSphs, and thedistribution of red horizontal branch (HB) stars is more concentrated towardthe centre than that of blue HB stars, suggesting that the galaxy containscomplex stellar populations. Boo I and CVn I dSphs show the elongated anddistorted shapes. CVn II dSph has the smallest tidal radius as a Milky Waysatellite and has distorted shape, while Leo IV dSph shows less concentratedspherical shape. The simple stellar population of faint UFDs indicates that thegases in their progenitors were removed more effectively than those of brighterdSphs at an occurrence of their initial star formation. This is reasonable ifthe progenitors of UFDs belong to the less massive halos than those of brighterdSphs.

The Stellar Populations and Structural Properties of Ultra Faint Dwarf Galaxies, Canes Venatici I, Bootes I, Canes Venatici II, and Leo IV