MIDIS: Unveiling the Role of Strong Ha-emitters during the Epoch of Reionization with JWST

By using the ultra-deep JWST/MIRI image at 5.6 μ m 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 M_⊙). These HAEs show a wide range of UV-β slopes, with a median value of β = -2.07±0.20 which broadly correlates with stellar mass. We estimate the ionizing photon production efficiency (ξ_ion,0) of these sources (assuming f_esc,LyC = 0%), which yields a median value log_10(ξ_ion,0/(Hz erg^-1)) = 25.49^+0.10_-0.12. We show that ξ_ion,0 positively correlates with EW_0(Hα) and specific star formation rate (sSFR). Instead ξ_ion,0 weakly anti-correlates with stellar mass and β. Based on the β values, we predict f_esc, LyC=4%^+2_-1, which results in log_10(ξ_ion/(Hz erg^-1)) = 25.52^+0.09_-0.08. Considering this and related findings from the literature, we find a mild evolution of ξ_ionwith redshift. Additionally, our results suggest that these HAEs require only modest escape fractions (f_esc, rel) of 6-14% to reionize their surrounding intergalactic medium. By only considering the contribution of these HAEs, we estimated their total ionizing emissivity (Ṅ_ion) as Ṅ_ion = 10^50.59 ± 0.45; s^-1Mpc^-3. When comparing their Ṅ_ion with non-Hα emitter 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.