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

Abstract

We make use of JWST medium-band and broadband NIRCam imaging, along with ultradeep MIRI 5.6 μ m 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 (H β + [O iii ]) emitters, based on their enhanced fluxes in the F430M and F444W filters, with EW _0 (H β +[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 μ m, indicating the simultaneous presence of a prominent H α emission line with EW _0 (H α ) ≃200–3000 Å. This is the first time that H α emission can be detected in individual galaxies at z > 7. The H α line, when present, allows us to separate the contributions of H β and [O iii ] to the (H β +[O iii ]) complex and derive H α -based star formation rates (SFRs). We find that in most cases [O iii ]/H β > 1. Instead, two galaxies have [O iii ]/H β < 1, indicating that the NIRCam flux excess is mainly driven by H β . 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 ${\mathrm{log}}_{10}({\rho }_{{\mathrm{SFR}}_{{\rm{H}}\alpha }}/({M}_{\odot }\,{\mathrm{yr}}^{-1}\,{\mathrm{Mpc}}^{-3}))\simeq -2.35$ , which is about a quarter of the total value ( ${\mathrm{log}}_{10}({\rho }_{{\mathrm{SFR}}_{\mathrm{tot}}}/({M}_{\odot }\,{\mathrm{yr}}^{-1}\,{\mathrm{Mpc}}^{-3}))\simeq -1.76$ ) at z ≃ 7–8. Therefore, the strong H α emitters likely had a significant role in reionization.

Keywords

• Galaxy formation
• Reionization
• Star formation
• Galaxy evolution
• Starburst galaxies