Population gradient in Sextans dSph: Comprehensive mapping of a dwarf
  galaxy by Suprime-Cam
  We present the deep and wide $V$ and $I_c$ photometry of the Sextans dwarf
spheroidal galaxy (dSph) taken by Suprime-Cam imager on the Subaru Telescope,
which extends out to the tidal radius. The colour-magnitude diagram (CMD)
reaches two magnitudes below the main sequence (MS) turn-off, showing a steep
red giant branch, blue and red horizontal branch (HB), sub-giant branch (SGB),
MS, and blue stragglers (BS). We construct the radial profile of each
evolutionary phase and demonstrate that blue HB stars are more spatially
extended, while red HB stars are more centrally concentrated than the other
components. The colour distribution of SGB stars also varies with the
galactocentric distance; the inner SGB stars shift bluer than those in the
outskirt. The radial differences in the CMD morphology indicate the existence
of the age gradient. The relatively younger stars ($\sim10$ Gyr) are more
centrally concentrated than the older ones ($\sim13$ Gyr). The spatial contour
maps of stars in different age bins also show that the younger population has
higher concentration and higher ellipticity than the older one. We also detect
the centrally concentrated bright BS stars, the number of which is consistent
with the idea that a part of these stars belongs to the remnant of a disrupted
star cluster discovered in the previous spectroscopic studies.
