MIDIS: Unveiling the Role of Strong Hα Emitters During the Epoch of Reionization with JWST

By using an ultradeep JWST/MIRI image at 5.6 μm in the Hubble eXtreme Deep Field, we constrain the role of strong Hα emitters (HAEs) during “cosmic reionization” at z ; 7–8. Our sample of HAEs is comprised of young (<35 Myr) galaxies, except for one single galaxy (≈300 Myr), with low stellar masses (10^9 Me). These HAEs show a wide range of rest-frame UV continuum slopes (β), with a median value of β = −2.15 ± 0.21, which broadly correlates with stellar mass. We estimate the ionizing photon production efficiency (ξion,0) of these sources (assuming fesc,LyC = 0%), which yields a median value log10(Eion,0/Hz erg^-1 ))= 25.50 +10 ion-12. We show that ξion,0 positively correlates with Hα equivalent width and specific star formation rate. Instead ξion,0 weakly anticorrelates with stellar mass and β. Based on the β values, we predict = - + f esc,LyC 4% 2 3 , which results in ( ( )) x - = - + log Hz erg 25.55 10 ion 1 0.13 0.11. Considering this and related findings from the literature, we find a mild evolution of ξion with redshift. Additionally, our results suggest that these HAEs require only modest escape fractions (fesc,rel) of 6%–15% to reionize their surrounding intergalactic medium. By only considering the contribution of these HAEs, we estimated their total ionizing emissivity (Nion) as Nion= 10^50.53+/-0.45 s^-1 MPC^-13. When comparing their Nion with non-HAE galaxies across the same redshift range, we find that that strong, young, and low-mass emitters may have played an important role during cosmic reionization