Ionizing radiation from young star-forming galaxies are thought to have played a primary role in the cosmic reionization. Knowing the amount of ionizing photons from galaxies has particular importance to understand the reionization process. Since ionizing photons are easily absorbed by intervening neutral hydrogen clouds, direct observation of ionizing radiation at z>6 is virtually impossible. Even at z~3 where there is a significant possibility of relatively transparent line of sight, very deep observation is required and escape fraction of ionizing photons remains highly uncertain to date. Here we report the results of Subaru/Suprime-Cam deep narrow-band imaging observation. The narrow-band filter was produced to optimally trace ionizing radiation from galaxies in the SSA22 proto-cluster at z=3.1. The unique wide field-of-view of Suprime-Cam enabled us to search ionizing photons from more than 100 spectroscopically confirmed galaxies. We detected ionizing radiation for about 10 galaxies, and some of them show significant offsets of ionizing radiation from non-ionizing UV emission and Ly-alpha images. We also found cases with larger observed flux density ratios of ionizing to non-ionizing UV radiations than those expected from population synthesis models with standard initial mass functions, and these might indicate the presence of 'metal-free' stellar populations.
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