STELLAR POPULATIONS OF Lyα EMITTERS AT z ∼ 6-7: CONSTRAINTS ON THE ESCAPE FRACTION OF IONIZING PHOTONS FROM GALAXY BUILDING BLOCKS

We investigate the stellar populations of Ly alpha emitters (LAEs) at z = 5.7 and 6.6 in a 0.65 deg(2) sky of the Subaru/XMM-Newton Deep Survey (SXDS) Field, using deep images taken with the Subaru/Suprime-Cam, United Kingdom Infrared Telescope/Wide Field Infrared Camera, and Spitzer/Infrared Array Camera (IRAC). We produce stacked multiband images at each redshift from 165 (z = 5.7) and 91 (z = 6.6) IRAC-undetected objects to derive typical spectral energy distributions (SEDs) of z similar to 6-7 LAEs for the first time. The stacked LAEs have as blue UV continua as the Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) z-dropout galaxies of similar M-UV, with a spectral slope beta similar to -3, but at the same time they have red UV-to-optical colors with detection in the 3.6 mu m band. Using SED fitting we find that the stacked LAEs have low stellar masses of similar to(3-10) x 10(7) M-circle dot, very young ages of similar to 1-3 Myr, negligible dust extinction, and strong nebular emission from the ionized interstellar medium, although the z = 6.6 object is fitted similarly well with high-mass models without nebular emission; inclusion of nebular emission reproduces the red UV-to-optical colors while keeping the UV colors sufficiently blue. We infer that typical LAEs at z similar to 6-7 are building blocks of galaxies seen at lower redshifts. We find a tentative decrease in the Lya escape fraction from z = 5.7 to 6.6, which may imply an increase in the intergalactic medium neutral fraction. From the minimum contribution of nebular emission required to fit the observed SEDs, we place an upper limit on the escape fraction of ionizing photons of f(esc)(ion) similar to 0.6 at z = 5.7 and similar to 0.9 at z = 6.6. We also compare the stellar populations of our LAEs with those of stacked HST/WFC3 z-dropout galaxies.