(Proposal ID) S15B-061I

(PI) Chiba, Masashi

(Proposal Title)
Probing faint substructures in the outer halo of the Andromeda galaxy

(Abstract)
{This is a resubmission of our previously accepted program, which was very 
incomplete due to the bad weather.} We propose a deep and wide-field survey of 
Andromeda's stellar halo using an HSC imager. This extremely wide-field camera 
with Subaru will reach a depth of 27.4 and 26.4 mag in g and i bands, 
respectively, to obtain numerous horizontal branch (HB) stars: we will 
identify and characterize a wealth of faint halo features through these HBs as 
expected from LambdaCDM models, whereas these were not detected by previous 
surveys based on the analysis of bright red giant branch (RGB) stars. This HSC 
survey will also allow us to find lower luminosity dwarf satellites and 
globular clusters through identification of member HBs than hitherto possible, 
thereby providing important insight into the true luminosity and spatial 
distributions of these objects. Our ultimate goal is to set advanced 
constraints on the past merging/accretion history of Andromeda's halo, in 
comparison with the Milky Way, and for this purpose we will trace individual 
stellar streams through the calibration of their metallicities and ages based 
on the combined properties of both HB and RGB stars, where the latter stars 
are selected from the use of a narrow-band filter NB515 developed by our group.