  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 x 26.67 arcmin^2, far
beyond the tidal radius of Leo II (8.63 arcmin), down to the limiting magnitude
of V ~26, which is roughly 1 mag deeper than the turn-off 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 of globular cluster luminosity beyond the tidal radius. We
investigated the properties of the stellar population by means of the
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 (SGB) 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.
