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

By using the ultra-deep \textit{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 $z\simeq7-8$. Our sample of HAEs is comprised of young ($<35\;\rm Myr$) galaxies, except for one single galaxy ($\approx 300\;\rm Myr$), with low stellar masses ($\lesssim 10^{9}\;\rm M_{\odot}$). These HAEs show a wide range of UV-$\beta$ slopes, with a median value of $\beta = -2.15\pm0.21$ which broadly correlates with stellar mass. We estimate the ionizing photon production efficiency ($\xi_{ion,0}$) of these sources (assuming $f_{esc,LyC} = 0\%$), which yields a median value $\rm log_{10}(\xi_{ion,0}/(Hz\;erg^{-1})) = 25.50^{+0.10}_{-0.12}$. 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 $f_{esc, LyC}=4\%^{+3}_{-2}$, which results in $\rm log_{10}(\xi_{ion}/(Hz\;erg^{-1})) = 25.55^{+0.11}_{-0.13}$. 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.
Comment: 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)