  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.
