Narrowband Ca Photometry for Dwarf Spheroidal Galaxies. I.
  Chemostructural Study on Draco, Sextans, and Canes Venatici I
  A few dozen dwarf satellite galaxies of the Milky Way have been discovered,
which are often viewed as the remaining building blocks of our Galaxy. The
follow-up spectroscopy showed that dwarf galaxies have a sizeable spread in
their metallicities. Several scenarios were suggested to explain the
metallicity spread, which can be tested by the structural patterns of stellar
subpopulations with distinct metallicities. However, such chemical plus
structural examination, to which we refer to as "chemostructural study" is
hindered by the lack of stars with spectroscopic metallicity. Here we propose
the Ca$-$$by$ photometry as an alternative way to secure metallicities for a
2$-$3 orders of magnitude larger stellar sample than the spectroscopic sample
and thus enable us to perform a chemostructural study on dwarf galaxies. In
particular, we use the $hk$ index [$\equiv($Ca$-b)-(b-y)$], whose validity as a
photometric metallicity indicator (and crass insensitivity to age) for
red-giant-branch stars was upheld via Galactic globular clusters, and observe
three dwarf spheroidal galaxies$-$Draco, Sextans, and Canes Venatici I
(CVnI)$-$with Subaru/Suprime-Cam. We find that in all the galaxies the
metal-rich stellar populations are more centrally concentrated than the
metal-poor counterparts, suggesting that the central regions of the galaxies
underwent extended star formation. Such a negative radial metallicity gradient
for Sextans and CVnI opposes to the traditional spectroscopic results. We also
find that their metallicity distribution functions (MDFs) can be characterized
by a unimodal, skewed Gaussian shape with a metal-rich peak and a metal-poor
tail. We discuss their features in the chemostructure and MDF in terms of dwarf
galaxy formation theories.
