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

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