Nature and Nurture? Comparing Lyα Detections in UV-bright and Fainter [O III]+Hβ Emitters at z 8 with Keck/MOSFIRE

May 2023 • 2023ApJ...948...54R

Authors • Roberts-Borsani, Guido • Treu, Tommaso • Mason, Charlotte • Ellis, Richard S. • Laporte, Nicolas • Schmidt, Thomas • Bradac, Marusa • Fontana, Adriano • Morishita, Takahiro • Santini, Paola

Abstract • The 100% detection rate of Lyα emission in a sample of four luminous z ~ 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 Lyα 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 >4σ line emission in only two targets at redshifts z _Lyα = 7.1081 and 7.9622 with rest-frame EWs of 16-17 Å, ~1.5× weaker compared to their brighter counterparts. Thus, we find a reduced rate for Lyα 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 EW_0,Lyα distribution for z ~ 6 galaxies. However, even with an extreme EW_0,Lyα 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 Lyα 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.

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Takahiro Morishita

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