(Proposal ID) S13A-010

(PI) Koda, Jin

(Proposal Title)
Probing the Initial Mass Function in Extended Ultraviolet (XUV) Disks

(Abstract)
The GALEX UV satellite discovered tantalizing evidence of star formation (SF) 
far beyond the optical edge of galactic disks (i.e. extended UV disk, or XUV 
disk). This discovery provides a new opportunity for studying SF in the 
exceedingly low-density environment (~1/10 of typical SF density), spurring 
intense debate on the universality of the initial mass function (IMF) in such 
exceptional environments. Our pilot S-Cam study of M83�fs XUV disk led to 
support for the universal IMF at least in M83 (Koda et al. 2012). We propose 
an expansion of the pilot study by an order of magnitude, by observing 
additional 6 XUV disks in NA656(H��), B, I, and R-band with S-Cam in S13A. 
In conjunction with GALEX UV bands, these images will reveal the presence of 
O stars (H��; NA656-R) and O&B stars (UV) in stellar clusters -thus, 
constraining the high-mass end of the IMF. These multi-broadband images will 
enable us to determine the masses of the clusters with much improved accuracy 
(previously, relied only on R). The proposed observations will not only 
increase the statistical significance of our previous result, but also enable
 us to analyze the stochastic effect of IMF sampling in very low-mass -
clusters (10^(2+3) M_sun) - the regime about which there is much ongoing debate.