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We investigate the stellar populations of Lyman alpha emitters (LAEs) atz=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 Subaru/Suprime-Cam, UKIRT/WFCAM, andSpitzer/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 spectralenergy distributions (SEDs) of z~6-7 LAEs for the first time. The stacked LAEshave as blue UV continua as the HST/WFC3 z-dropout galaxies of similar Muv,with a spectral slope beta ~ -3, but at the same time they have redUV-to-optical colors with detection in the 3.6um band. Using SED fitting wefind that the stacked LAEs have low stellar masses of ~(3-10)*10^7 Msun, veryyoung ages of ~1-3 Myr, negligible dust extinction, and strong nebular emissionfrom the ionized interstellar medium, although the z=6.6 object is fittedsimilarly well with high-mass models without nebular emission; inclusion ofnebular emission reproduces the red UV-to-optical color while keeping the UVcolor sufficiently blue. We infer that typical LAEs at z~6-7 are buildingblocks of galaxies seen at lower redshifts. We find a tentative decrease in theLyman alpha escape fraction from z=5.7 to 6.6, which may imply an increase inthe intergalactic medium neutral fraction. From the minimum contribution ofnebular emission required to fit the observed SEDs, we place an upper limit onthe escape fraction of ionizing photons to be f_esc^ion~0.6 at z=5.7 and ~0.9at z=6.6. We also compare the stellar populations of our LAEs with that ofstacked HST/WFC3 z-dropout galaxies.

Stellar Populations of Lyman Alpha Emitters at z~6-7: Constraints on the Escape Fraction of Ionizing Photons from Galaxy Building Blocks