Nature and Nurture? Comparing Ly$\alpha$ Detections in UV-Bright and Fainter [O III]+H$\beta$ Emitters at $z\sim 8$ With Keck/MOSFIRE

Abstract

The 100% detection rate of Ly$\alpha$ emission in a sample of four luminous $z\sim 8$ 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 $z_{\mathrm{Ly}\alpha }$=7.1081 and $z_{\mathrm{Ly}\alpha }$=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 $0.40_{-0.25}^{+0.30}$ compared to $1.00_{-0.44}^{+0.00}$ for more luminous examples. The lower rate agrees with predictions from simulations of a mostly neutral IGM and an intrinsic EW${}_{0,\mathrm{Ly}\alpha }$ distribution for $z\sim 6$ galaxies. However, even with an extreme EW${}_{0,\mathrm{Ly}\alpha }$ 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.

15 pages, 4 figures, 2 tables. Accepted for publication in ApJ (in press), uploaded to match accepted version

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• Detection of the 2175Å UV Bump at z>7: Evidence for Rapid Dust Evolution in a Merging Reionisation-Era Galaxy
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• Deep JWST spectroscopy of galaxies in a candidate ionized bubble at $z=8.7$: probing reionization at pMpc scales with Ly$\alpha$ emission