(Proposal ID) S17B-047

(PI) Harikane, Yuichi

(Proposal Title)
Understanding Reionizing Sources with Newly-Identified IRAC Excess Galaxies

(Abstract)
We propose Keck/MOSIFRE spectroscopy for eight objects showing Spitzer/IRAC 
color excesses at z>5.5 to investigate statistical properties of ionizing 
sources for the first time at the epoch of reionization. Our targets include 
a plausible Pop-III galaxy candidate with low [O III]5007/H alpha ratio and 
large Ly alpha equivalent width of EW_0~220 Angstrom at z_spec=5.712. 
These objects are newly identified in the Subaru/Hyper-Suprime Cam (HSC) 
survey and the Spitzer Large Area Survey with HSC (SPLASH), whose area is x10 
larger than the literature. The proposed observations allow us to measure 
strength of high-ionization emission lines such as C IV 1549, He II 1640, or 
C III 1909 for four spectroscopically confirmed targets at z_spec=5.647-5.712, 
and to confirm redshifts of four z>7 candidates by Ly alpha. We will 
investigate a UV luminosity dependence of the ionizing photon emission rate, 
xi_{ion}, and the presence of prominent He II emission in the PopIII candidate,
 to discuss the contribution of each ionizing source (star-forming galaxies, 
AGNs, and Pop-III) to the reionization. These observations will greatly 
increase our understanding of ionizing sources, and provide ideal targets for 
near-future JWST observations.