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

Abstract

By using an ultradeep JWST/MIRI image at 5.6 μ m in the Hubble eXtreme Deep Field, we constrain the role of strong H α 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 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 ( ξ _ion,0 ) of these sources (assuming f _esc,LyC = 0%), which yields a median value ${\mathrm{log}}_{10}({\xi }_{\mathrm{ion},0}/(\mathrm{Hz}\ {\mathrm{erg}}^{-1}))={25.50}_{-0.12}^{+0.10}$ . We show that ξ _ion,0 positively correlates with H α equivalent width and specific star formation rate. Instead ξ _ion,0 weakly anticorrelates with stellar mass and β . Based on the β values, we predict ${f}_{\mathrm{esc},\mathrm{LyC}}=4{ \% }_{-2}^{+3}$ , which results in ${\mathrm{log}}_{10}({\xi }_{\mathrm{ion}}/(\mathrm{Hz}\ {\mathrm{erg}}^{-1}))={25.55}_{-0.13}^{+0.11}$ . Considering this and related findings from the literature, we find a mild evolution of ξ _ion with redshift. Additionally, our results suggest that these HAEs require only modest escape fractions ( f _esc,rel ) of 6%–15% to reionize their surrounding intergalactic medium. By only considering the contribution of these HAEs, we estimated their total ionizing emissivity ( ${\dot{N}}_{\mathrm{ion}}$ ) as ${\dot{N}}_{{\rm{ion}}}={10}^{50.53\pm 0.45}\,{{\rm{s}}}^{-1}{{\rm{Mpc}}}^{-3}$ . When comparing their ${\dot{N}}_{\mathrm{ion}}$ 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.

Keywords

• Galaxy formation
• Reionization
• Starburst galaxies
• Galaxy evolution
• High-redshift galaxies
• Star formation