Detections of Lyman Continuum from Star-forming Galaxies at Z ∼ 3 through Subaru/suprime-cam Narrow-band Imaging

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%. Subject headings: galaxies: evolution — galaxies: high-redshift — cosmology: observations — intergalactic medium — diffuse radiation