High Equivalent Width of H alpha plus [N II] Emission in z similar to 8 Lyman-break Galaxies from IRAC 5.8 mu m Observations:Evidence for Efficient Lyman-continuum Photon Production in the Epoch of Reionization

We measure, for the first time, the median equivalent width (EW) of H alpha+[N II] in star-forming galaxies at z similar to 8. Our estimate leverages the unique photometric depth of the Spitzer/IRAC 5.8 mu m band mosaics (probing approximate to 5500-7100 angstrom at z similar to 8) of the GOODS Reionization Era Wide Area Treasury from Spitzer (GREATS) program. We median-stacked the stamps of 102 Lyman-break galaxies in the 3.6, 4.5, 5.8, and 8.0 mu m bands, after carefully removing potential contamination from neighboring sources. We infer an extreme rest-frame EW0(H alpha+[N II]) = 2328(-1127)(+1326) angstrom from the measured red [3.6] - [5.8] = 0.82 +/- 0.27 mag, consistent with young (less than or similar to 10(7) yr) average stellar population ages at z similar to 8. This implies an ionizing photon production efficiency of log(xi(ion,0)/erg Hz(-1)) = 25.97(-0.28)(+0.18). Such a high value for photoproduction, similar to the highest values found at z less than or similar to 4, indicates that only modest escape fractions f(esc) less than or similar to 0.3 (at 2 sigma) are sufficient for galaxies brighter than M-UV < -18 mag to reionize the neutral hydrogen at z similar to 8. This requirement is relaxed even more to f(esc)