(Proposal ID) S15B-014

(PI) Koda, Jin

(Proposal Title)
The Building History of XUV disks with TRGB Archaeology

(Abstract)
We propose HSC gri-band imaging of two extended ultraviolet (XUV) disks to 
study resolved stellar populations. The two will be selected from the complete 
sample of 4 prototype XUV disks of the closest (m-M <29; <6.3Mpc) and thus 
of largest (~20-60 arcmin). This project probes the building history of the 
XUV disks using archeological stellar populations, especially the tip of red 
giant branch (TRGB) stars (age ~ 2-14 Gyr). Their {presence} and {distribution}
 over the XUV disks will reveal any star formation (SF) occurring over the 
past ~2 Gyr, ~4-6 Gyr, and beyond - i.e., the epochs preceding the recent 
(UV-traced) state of SF. Their color depends strongly on metallicity, thus 
providing an additional measure of star-gas recycling during the evolution of 
the XUV disks. We will also detect young & massive main sequence stars (<100 
Myr) and He-burning stars (100-500 Myr). Comparing various generations of 
stars, in terms of number densities and spatial distributions, will reveal 
the much-unexplored SF history in the XUV disks. The HSC permits deep imaging 
over the huge XUV disks, plus sufficient surrounding areas for statistical 
estimation of background contamination, as well as a photometric consistency 
for TRGB identification and gri-based star/galaxy separation.