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We make use of \textit{JWST} medium and broad-band NIRCam imaging, along withultra-deep MIRI $5.6 \rm \mu m$ imaging, in the Hubble eXtreme Deep Field (XDF)to identify prominent line emitters at $z\simeq 7-8$. Out of a total of 58galaxies at $z\simeq 7-8$, we find 18 robust candidates ($\simeq$31\%) for(H$\beta$ + [OIII]) emitters, based on their enhanced fluxes in the F430M andF444W filters, with EW$_{0}$(H$\beta$ +[OIII]) $\simeq 87 - 2100$ {\AA}. Amongthese emitters, 16 lie in the MIRI coverage area and 12 exhibit a clear fluxexcess at $5.6 \, \rm \mu m$, indicating the simultaneous presence of aprominent H$\alpha$ emission line with EW$_{0}$(H$\alpha$) $\simeq 200-3000${\AA}. This is the first time that H$\alpha$ emission can be detected inindividual galaxies at $z>7$. The H$\alpha$ line, when present, allows us toseparate the contributions of H$\beta$ and [OIII] to the (H$\beta$ +[OIII])complex, and derive H$\alpha$-based star formation rates (SFRs). We find thatin most cases [OIII]/H$\beta > 1$. Instead, two galaxies have [OIII]/H$\beta <1$, indicating that the NIRCam flux excess is mainly driven by H$\beta$. Thiscould potentially imply extremely low metallicities. Most prominent lineemitters are very young starbursts or galaxies on their way to/from thestarburst cloud. They make for a cosmic SFR density $\rmlog_{10}(\rho_{SFR_{H\alpha}}) \simeq -2.35$, which is about a quarter of thetotal value ($\rm log_{10}(\rho_{SFR_{tot}}) \simeq -1.76$) at $z\simeq 7-8$.Therefore, the strong H$\alpha$ emitters likely had a significant role inreionization.

astrophysical journal/the astrophysical journal

MIDIS: Strong (Hb + [OIII]) and Ha emitters at redshift $z \simeq 7-8$  unveiled with JWST/NIRCam and MIRI imaging in the Hubble eXtreme Deep Field  (XDF)