Detections of Lyman Continuum from Star-forming Galaxies at z~3 Through Subaru/Suprime-Cam Narrow-band Imaging

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 of the SSA22 proto-cluster region at z=3.09, using a special narrow-band filter to optimally trace Lyman continuum (LyC) from galaxies at z~3. The unique wide field-of-view of Suprime-Cam enabled us to search for ionizing photons from 198 galaxies (73 Lyman break galaxies (LBGs) and 125 Ly-alpha emitters (LAEs)) with spectroscopically measured redshifts z~3.1. We detected LyC from 7 LBGs, as well as from 10 LAE candidates. Some of the detected galaxies show significant spatial offsets of LyC from non-ionizing UV emission. For some LBGs the observed non-ionizing UV to LyC flux density ratios are smaller than values expected from population synthesis models with a standard Salpeter initial mass function (IMF) with moderate dust attenuation (which is suggested from the observed UV slopes), even if we assume very transparent IGM along the sightlines of these objects. This implies an intrinsically bluer spectral energy distribution, e.g, that produced by a top-heavy IMF, for these LBGs. The observed flux desity ratios of non-ionizing UV to LyC of 7 detected LBGs range from 2.4 to 23.8 and the median is 6.6. The observed flux density ratios of the detected LAEs are even smaller than LBGs, if they are truly at z~3.1. We find that the median value of the flux density ratio for the deteced LBGs suggest that their escape fractions is likely to be higher than 4%, if the LyC escape is isotropic. The results imply that some of the LBGs in the proto-cluster at z~3 have the escape fraction significantly higher than that of galaxies (in a general field) at z~1 studied previously.