(Proposal ID) S15A-017

(PI) Koda, Jin

(Proposal Title)
 The Building History of XUV disks of M83& NGC2403 with TRGB Archaeology

(Abstract)
We propose deep HSC g & i-band imaging of two extended ultraviolet (XUV) disks 
of M83 and NGC2403. These galaxies have the prototype XUV disks with the 
largest size (~ 1 deg and ~ 30 arcmin). The Subaru HSC permits unprecedentedly 
deep imaging over these gigantic XUV disks, including sufficient surrounding 
areas which are used for sky subtraction and statistical estimation of 
background contamination. 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. In addition, we will 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.