Nature and Nurture? Comparing Ly$\alpha$ Detections in UV-Bright and Fainter [O III]+H$\beta$ Emitters at $z\sim8$ With Keck/MOSFIRE

Abstract

The 100% detection rate of Ly$\alpha$ emission in a sample of four luminous $z\sim8$ 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 $z_{\rm Ly\alpha}$=7.1081 and $z_{\rm Ly\alpha}$=7.9622 with rest-frame EWs of 16-17 A, $\sim$1.5$\times$ weaker compared to their brighter counterparts. Thus we find a reduced rate for Ly$\alpha$ emission of $0.40^{+0.30}_{-0.25}$ compared to $1.00^{+0.00}_{-0.44}$ for more luminous examples. The lower rate agrees with predictions from simulations of a mostly neutral IGM and an intrinsic EW$_{0,\rm Ly\alpha}$ distribution for $z\sim6$ galaxies. However, even with an extreme EW$_{0,\rm Ly\alpha}$ model, it is challenging to match the detection rate for the luminous objects. SED-fitting of our fainter sample indicates young and star-forming systems, albeit with less extreme SFRs 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 environment effects. Further studies with JWST may be required to resolve the physical nature of this puzzling population.