(Proposal ID) S15A-070

(PI) Yoon, Suk-Jin

(Proposal Title)
 A Chemo-structural Study on Galactic Building Blocks

(Abstract)
Over a dozen ultra-faint dwarf galaxies (UFDs) have recently been discovered, 
which are believed to be remaining building blocks of the Milky Way (MW). The 
follow-up stellar spectroscopy reveals a sizable metallicity spread within 
each UFD. Various scenarios were put forward to explain the chemical 
inhomogeneity, and they can readily be tested by examining the structural 
feature of stellar populations as a function of metallicity. However, such 
chemical plus structural investigation, referred here to "chemo-structural 
study'' has so far been hindered by the lack of stars with measured 
spectroscopic metallicity. Here we present an alternative way to obtain 
stellar metallicity for over 1000 times larger sample than spectroscopy, and 
thus enable to perform the first chemo-structural study on MW satellites. 
For two newly-found UFDs and three dwarf spheroidal, we propose the Ca-by 
photometry whose validity as a metallicity indicator was confirmed via MW 
globular clusters. The data will allow an in-depth study on the spatial 
distribution of sub-populations with distinct metallicities and thus to 
address the nature and origin of the MW companions. To this aim, we ask for 
3 nights. We are in possession of the new Ca-by filters for Subaru/Suprime-cam.