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

Abstract

By using the ultra-deep \textit{JWST}/MIRI image at 5.6 $\hat{I}¼m$ in the Hubble eXtreme Deep Field, we constrain the role of strong H$\hat{I}\pm$-emitters (HAEs) during Cosmic Reionization at $z\simeq 7-8$. Our sample of HAEs is comprised of young ($<35\mathrm{Myr}$) galaxies, except for one single galaxy ($\approx 300\mathrm{Myr}$), with low stellar masses ($\lesssim 10^9{\mathrm{M}}_{\odot }$). These HAEs show a wide range of UV-${\hat{I}}^2$ slopes, with a median value of ${\hat{I}}^2=-2.15\pm 0.21$ which broadly correlates with stellar mass. We estimate the ionizing photon production efficiency ($\hat{I}{¾}_{ion,0}$) of these sources (assuming $f_{esc,LyC}=0\%$), which yields a median value $\mathrm{lo}{\mathrm{g}}_{10}(\hat{\mathrm{I}}{¾}_{\mathrm{ion},0}/(\mathrm{Hz}\mathrm{er}{\mathrm{g}}^{-1}))=25.50_{-0.12}^{+0.10}$. We show that $\hat{I}{¾}_{ion,0}$ positively correlates with EW${}_0$(H$\hat{I}\pm$) and specific star formation rate (sSFR). Instead $\hat{I}{¾}_{ion,0}$ weakly anti-correlates with stellar mass and ${\hat{I}}^2$. Based on the ${\hat{I}}^2$ values, we predict $f_{esc,LyC}=4{\%}_{-2}^{+3}$, which results in $\mathrm{lo}{\mathrm{g}}_{10}(\hat{\mathrm{I}}{¾}_{\mathrm{ion}}/(\mathrm{Hz}\mathrm{er}{\mathrm{g}}^{-1}))=25.55_{-0.13}^{+0.11}$. Considering this and related findings from the literature, we find a mild evolution of $\hat{I}{¾}_{ion}$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}}_{ion}$) as ${\dot{N}}_{ion}=10^{50.53\pm 0.45};{\text{s}}^{-1}{\text{Mpc}}^{-3}$. When comparing their ${\dot{N}}_{ion}$ with "non-H$\hat{I}\pm$ 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.

21 pages, 11 Figures, 1 table. Accepted for publication in ApJ. Updated version 13/06/2024 -- found a mistake in the reported eq. 2 (now corrected)