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

Abstract

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) <= 0.1 when considering galaxies brighter than M-UV approximate to -13 mag, consistent with recent luminosity functions and as typically assumed in studies addressing reionization. These exceptional results clearly indicate that galaxies can be the dominant source of reionizing photons, and provide us with an exciting glimpse into what we might soon learn about the early universe, and particularly about the reionization epoch, from forthcoming JWST/MIRI and NIRCam programs.