MIDIS: Strong (Hβ+[O iii]) and Hα Emitters at Redshift z ≃ 7-8 Unveiled with JWST NIRCam and MIRI Imaging in the Hubble eXtreme Deep Field

We make use of JWST medium-band and broadband NIRCam imaging, along with ultradeep MIRI 5.6 μm imaging, in the Hubble eXtreme Deep Field to identify prominent line emitters at z ≃ 7-8. Out of a total of 58 galaxies at z ≃ 7-8, we find 18 robust candidates (≃31%) for (Hβ + [O iii]) emitters, based on their enhanced fluxes in the F430M and F444W filters, with EW₀(Hβ +[O iii]) ≃87-2100 Å. Among these emitters, 16 lie in the MIRI coverage area and 12 exhibit a clear flux excess at 5.6 μm, indicating the simultaneous presence of a prominent Hα emission line with EW₀(Hα) ≃200-3000 Å. This is the first time that Hα emission can be detected in individual galaxies at z > 7. The Hα line, when present, allows us to separate the contributions of Hβ and [O iii] to the (Hβ +[O iii]) complex and derive Hα-based star formation rates (SFRs). We find that in most cases [O iii]/Hβ > 1. Instead, two galaxies have [O iii]/Hβ < 1, indicating that the NIRCam flux excess is mainly driven by Hβ. Most prominent line emitters are very young starbursts or galaxies on their way to/from the starburst cloud. They make for a cosmic SFR density log 10 ( ρ SFR H α / ( M ⊙ yr − 1 Mpc − 3 ) ) ≃ − 2.35 , which is about a quarter of the total value ( log 10 ( ρ SFR tot / ( M ⊙ yr − 1 Mpc − 3 ) ) ≃ − 1.76 ) at z ≃ 7-8. Therefore, the strong Hα emitters likely had a significant role in reionization.