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 EW0,Lyα distribution for z ~ 6 galaxies. However, even with an extreme EW0,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.


IPAC Authors


Takahiro Morishita

Assistant Scientist