A Search for <i>z</i> = 5 H<i>α</i> and H<i>β</i> + [O <scp>iii</scp>] Dual-line Emitting Galaxies in the JWST CEERS Field: Implications for the Active Galactic Nucleus Abundance

Abstract The James Webb Space Telescope (JWST) has enabled us to uncover faint galaxies and active galactic nuclei (AGNs) in the early Universe. Leveraging the filter combination used in the Cosmic Evolution Early Release Science Survey program, we perform an extensive photometric search of galaxies emitting strong H β + [O iii ] and H α lines. The redshift range of the galaxies is limited to 5.03 ≤ z ≤ 5.26 by requiring photometric excess in NIRCam’s F277W and F410M images. A total of 259 H β + [O iii ] and H α dual-line emitters are found over the absolute UV magnitude −22 ≲ M UV ≲ −17, with a mean rest-frame equivalent width of 990 Å for H β + [O iii ] and 1050 Å for H α . This population accounts for ∼40% of the Lyman-break galaxies at this redshift range. Intriguingly, there are 58 objects (22% of the whole sample) that exhibit compact morphology at the rest-UV or optical wavelength. Assuming that these compact dual-line emitters are dominated by AGNs, their bolometric luminosities are in the range of 2 × 10 43 ≲ L bol /(erg s −1 ) ≲ 3 × 10 44 . Their number density is 2 dex higher than the extrapolation from UV-selected luminous quasars, which is in good agreement with previous JWST studies of broad-line AGNs, requiring a ∼10% AGN duty cycle. Moreover, our dual-line emitter sample reaches the faint end of the H α and [O iii ] luminosity functions down to ≲10 42 erg s −1 . Spectroscopic follow-up observations are planned in a JWST Cycle 3 program, in which we aim to confirm their nature, characterize their black hole activity, and construct their mass distribution at 10 6 ≲ M BH / M ⊙ ≲ 10 8 .