Narrowband Ca photometry for dwarf spheroidal galaxies

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, 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 [=(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.