MIDIS: The Relation between Strong (Hβ + O iii) Emission, Star Formation, and Burstiness around the Epoch of Reionization

We investigate the properties of strong (Hβ + O iii) emitters before and after the end of the “Epoch of Reionization” from z = 8 to z = 5.5. We make use of ultradeep JWST/NIRCam imaging in the parallel field (P2) of the MIRI Deep Imaging Survey (MIDIS) in the Hubble eXtreme Deep Field (H-XDF), in order to select prominent (Hβ + O iii) emitters (with rest-frame equivalent width (EW0) ≳ 100 Å) at z = 5.5-7, based on their flux density enhancement in the F356W band with respect to the spectral energy distribution continuum. We complement our selection with other (Hβ + O iii) emitters from the literature at similar and higher (z = 7−8) redshifts. We find (nonindependent) anticorrelations between EW0(Hβ + O iii) and both galaxy stellar mass and age, in agreement with previous studies, and a positive correlation with specific star formation rate (sSFR). On the SFR-M ⋆ plane, the (Hβ + O iii) emitters populate both the star formation main sequence and the starburst region, which become indistinguishable at low stellar masses ( log 10 ( M ⋆ ) < 7.5 ). We find tentative evidence for a nonmonotonic relation between EW0(Hβ + O iii) and SFR, such that both parameters correlate with each other at SFR ≳ 1 M ⊙ yr−1, while the correlation flattens out at lower SFRs. This suggests that low metallicities producing high EW0(Hβ + O iii) could be important at low SFR values. Interestingly, the properties of the strong emitters and other galaxies (33% and 67% of our z = 5.5-7 sample, respectively) are similar, including, in many cases, high sSFR. Therefore, it is crucial to consider both emitters and nonemitters to obtain a complete picture of the cosmic star formation activity around the Epoch of Reionization.