Nature of High Equivalent Width Emitters in the Epoch of Reionization Revealed by JWST Medium-band Imaging

Extreme emission line galaxies (EELGs) at high redshifts are considered key contributors to cosmic reionization at z > 6 due to their higher ionization efficiencies. We have identified 119 H β + [O iii ] emitters at z ∼ 7 selected by a flux excess in the medium-band filter F410M in the public James Webb Space Telescope Cycle-1 fields. Our emitters exhibit a wide range in rest-frame H β + [O iii ] equivalent width (EWs), 420 < EW 0 / Å < 6850 (with the median value of ∼1700 Å). Among them, 19 are EW 0 > 3000 / Å, which represent extreme populations even in the context of recent findings with JWST. They are characterized by (i) low stellar mass (∼3 × 10 7 M ⊙ ), (ii) blue colors ( β UV ∼ −2.2), and (iii) low dust attenuation ( A V ∼ 0.1 mag). We discuss the physical mechanisms responsible for the observed high rest-frame H β + [O iii ] EWs, including (1) photoionization by active galactic nucleus (AGN), (2) stellar photoionization in the vicinity of H ii regions, and (3) radiative shocks powered by outflows either from AGN or massive stars. Notably, we find 13 emitters with spatially offset H β + [O iii ] emission compared to the UV and stellar components. Given the absence of obvious signatures of actively accreting black holes, these emitters are likely under strong feedback-driven winds from massive stars. Lastly, we report a unique overdensity of EELGs in one of the observed fields. The discovery of such a “star-bursting” overdensity supports the idea that large ionizing bubbles formed around some EEGLs in the early Universe.