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

TL;DR

This study identifies and characterizes extreme emission line galaxies at z~7 using JWST data, revealing their physical properties, potential ionization mechanisms, and their role in early cosmic reionization.

Contribution

First identification of a large sample of high-EW EELGs at z~7 with detailed analysis of their properties and implications for reionization.

Findings

119 Hβ + [OIII] emitters at z~7 identified

Presence of 19 extreme EELGs with EW > 3000 Å

Discovery of a star-bursting overdensity of EELGs

Abstract

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$\beta$ + [OIII] emitters at $z\sim7$ 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$\beta$ + [OIII] equivalent width (EWs), 420 $<$ EW$_{0}$/\r{A} $<$ 6850 (with the median value of $\sim1700$ \r{A}). Among them, 19 are EW$_{0}$ $>$ 3000 \r{A}, which represent extreme populations even in the context of recent findings with JWST. They are characterized by (i) low stellar mass ($\sim 3\times10^{7}$ $\mathrm{M_{\odot}}$), (ii) blue colors ($\beta_{\rm UV}\sim -2.2$), and (iii) low dust attenuation ($A_{\mathrm{V}}\sim 0.1$ mag). We discuss the physical mechanisms responsible for the observed high rest-frame H$\beta$ + [OIII] EWs, including (1) photoionization by AGN, (2) stellar photoionization in the vicinity of HII regions, and (3) radiative shocks powered by outflows either from AGN or massive stars. Notably, we find 13 emitters with spatially offset H$\beta$ + [OIII] 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.