Wide-Field Survey around Local Group Dwarf Spheroidal Galaxy Leo II: Spatial Distribution of Stellar Content

Abstract

We carried out a wide-field V, I imaging survey of the Local Group dwarf spheroidal galaxy Leo II using the Subaru Prime Focus Camera on the 8.2 m Subaru Telescope. The survey covered an area of 26.67 × 26.67 arcmin2, far beyond the tidal radius of Leo II (8.63'), down to the limiting magnitude of V 26, which is roughly 1 mag deeper than the turnoff point of the main-sequence stars of Leo II. Radial number density profiles of bright and faint red giant branch (RGB) stars were found to change their slopes at around the tidal radius, and extend beyond the tidal radius with shallower slopes. A smoothed surface brightness map of Leo II suggests the existence of a small substructure (4 × 2.5 arcmin2, 270 × 170 pc 2 in physical size) of globular cluster luminosity beyond the tidal radius. We investigated the properties of the stellar population by means of a color-magnitude diagram. The horizontal branch (HB) morphology index shows a radial gradient in which red HB stars are more concentrated than blue HB stars, which is common to many Local Group dwarf spheroidal galaxies. The color distribution of RGB stars around the mean RGB sequence shows a larger dispersion at the center than in the outskirts, indicating a mixture of stellar populations at the center and a more homogeneous population in the outskirts. Based on the age estimation using subgiant branch stars, we found that although the major star formation took place ~8 Gyr ago, a considerable stellar population younger than 8 Gyr is found at the center; such a younger population is insignificant in the outskirts. The following star formation history is suggested for Leo II. Star-forming activity occurred more than ~8 Gyr ago throughout the galaxy at a modest star formation rate. The star-forming region gradually shrank from the outside toward the center, and star-forming activity finally dropped to ~0 by ~4 Gyr ago, except for the center, where a small population younger than 4 Gyr is present.