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

The JWST is revolutionizing the study of high-redshift galaxies by providing for the first time a high-sensitivity view of the early Universe at infrared wavelengths, both with its Near Infrared Camera (NIRCam) and Mid Infrared Instrument (MIRI). In this paper, we make use of medium and broad-band NIRCam imaging, as well as ultra-deep MIRI 5.6 microns imaging, in the Hubble eXtreme Deep Field (XDF) 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 prominent (Hb + [OIII]) emitters, based on their enhanced fluxes in the F430M and F444W filters, with rest-frame EW(Hb + [OIII]) ~ 87 - 2100 A. Among these emitters, 16 lie on the MIRI coverage area and 12 show a clear flux excess at 5.6 microns, indicating the simultaneous presence of a prominent Ha emission line with rest-frame EW(Ha) ~ 200 - 3000 A. This is the first time that Ha emission can be detected in individual galaxies at z>7. The Ha line, when present, allows us to separate the contributions of the Hb and [OIII] emission lines to the (Hb + [OIII]) complex and derive Ha-based star formation rates (SFRs). We find that in some cases [OIII]/Hb > 1, suggesting low metallicities, but a few have [OIII]/Hb < 1, so the NIRCam flux excess is mainly driven by Hb. The vast majority of prominent line emitters are very young starbursts or galaxies on their way to/from the starburst cloud. They make for a cosmic SFR density log10(SFRD_Ha / Msun yr^-1 Mpc^-3) ~ 2.35, which is about a third of the total value at z ~ 7-8. Therefore, the strong Ha emitters likely had an important role in reionization.Comment: 15 pages, 9 figures. Submitted to Ap