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

TL;DR

This study compares Lyα emission in UV-bright and fainter galaxies at z~8 to understand whether their ionizing properties or environment influence early universe reionization, revealing that luminosity correlates with higher Lyα detection rates.

Contribution

It provides the first comparative analysis of Lyα emission between luminous and fainter z~8 galaxies, highlighting the roles of intrinsic properties and environment in early reionization.

Findings

Luminous galaxies show higher Lyα detection rates than fainter ones.

Detection rate in fainter galaxies is about 0.40, lower than 1.00 in luminous galaxies.

Results suggest both intrinsic ionizing properties and environment affect Lyα visibility.

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.