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Submitted to ApJL Preprint typeset using L ATEX style emulateapj v. 03/07/07 DETECTIONS OF LYMAN CONTINUUM FROM STAR-FORMING GALAXIES AT Z ∼ 3 THROUGH SUBARU/SUPRIME-CAM NARROW-BAND IMAGING 1

By I. Iwata, A. K. Inoue, Y. Matsuda, H. Furusawa, T. Hayashino, K. Kousai, M. Akiyama, T. Yamada and J. -m. Deharveng

Abstract

Knowing the amount of ionizing photons from young star-forming galaxies is of particular importance to understanding the reionization process. Here we report initial results of Subaru/Suprime-Cam deep imaging observation with a special narrow-band filter to optimally trace ionizing radiation from galaxies at z> 3. The unique wide field-of-view of Suprime-Cam enabled us to search for ionizing photons from 198 galaxies with spectroscopically measured redshifts z ≃ 3.1. We detected ionizing radiation from 7 Lyman break galaxies (LBGs), as well as from 10 Ly-α emitter (LAE) candidates. Some of the detected galaxies show significant offsets of ionizing radiation from non-ionizing UV emission. As an average of the 7 detected LBGs, the observed flux density ratio of non-ionizing UV to ionizing radiation is estimated to be 4.9, which is smaller than values expected from population synthesis models with a standard Salpeter initial mass function (IMF) and dust attenuation. This implies an intrinsically bluer spectral energy distribution, e.g, that produced by a top-heavy IMF, for these LBGs. The observed flux density ratios of the detected LAEs are even smaller than those expected from a top-heavy IMF and QSOs if they are truly at z ≃ 3.1. We find that the average escape fraction of ionizing photons for the detected LBGs should be higher than 15%

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