Nature and Nurture? Comparing Ly alpha Detections in UV-bright and Fainter [O III] plus H beta Emitters at z similar to 8 with Keck/MOSFIRE

The 100% detection rate of Ly alpha emission in a sample of four luminous z similar to 8 galaxies with red Spitzer/IRAC colors suggests objects with unusual ionizing capabilities that created early ionized bubbles in a neutral era. Whether such bubbles reflect enhanced ionizing properties (nature) or an overdense environment (nurture), however, remains unclear. Here we aim to distinguish between these hypotheses via a search for Ly alpha emission in five fainter galaxies drawn from the CANDELS-GOODS fields using a similar IRAC excess and UV magnitudes that should reflect reduced clustering effects. Using Keck/MOSFIRE we tentatively detect >4 sigma line emission in only two targets at redshifts z(Ly alpha) = 7.1081 and 7.9622 with rest-frame EWs of 16-17 angstrom, similar to 1.5x weaker compared to their brighter counterparts. Thus, we find a reduced rate for Lya emission of 0.40(-0.25)(+0.30) compared to 1.00(-0.44)(+0.00) for more luminous examples. The lower rate agrees with predictions from simulations of a mostly neutral intergalactic medium and an intrinsic EW0, Ly alpha distribution for z similar to 6 galaxies. However, even with an extreme EW0, Ly alpha model, it is challenging to match the detection rate for the luminous objects. Spectral energy distribution fitting of our fainter sample indicates young and star-forming systems, albeit with less extreme star formation rates and ionization parameters compared to their luminous counterparts. The enhanced Ly alpha rate in luminous galaxies is thus likely a byproduct of both extreme ionizing properties as well as environmental effects. Further studies with JWST may be required to resolve the physical nature of this puzzling population.