Extreme Emission Line Galaxies in CEERS Are Powered by Star Formation, not AGN

Kelcey Davis, Madisyn Brooks, Jonathan R. Trump, Vital Fernández, Taylor A. Hutchison, Rebecca L. Larson, Anthony J. Taylor, Elizabeth J. McGrath, Guillermo Barro, Anton M. Koekemoer, Pablo Arrabal Haro, Mark Dickinson, Bren E. Backhaus, Nikko J. Cleri, Steven L. Finkelstein, Ananya Ganapathy, Raymond C. Simons, Ricardo O. Amorín, Alexander de la Vega, Norman A. Grogin, Michaela Hirschmann, Weida Hu, Jarrett L. Johnson, Jeyhan S. Kartaltepe, Dale Kocevski, Mario Llerena, Ray A. Lucas, Madeline A. Marshall, Fabio Pacucci, Laura Pentericci, Phoebe R. Upton Sanderbeck

First listed 2026-02-26 | Last updated 2026-02-26

Abstract

We present a spectroscopic study of photometrically identified extreme emission-line galaxies (EELGs) with observed-frame equivalent widths (EWs) >5000 A of either H alpha or H beta + [OIII] in the CEERS legacy deep field utilizing JWST NIRSpec spectroscopy from the CAPERS, RUBIES, THRILS and CEERS surveys. This master sample allows for performance tests of photometric selections and unveils what types of sources, either AGN or young star formation, were producing excessive ionizing radiation in the early Universe. We identify AGN through broad H alpha emission-lines and report 6 new broad-line AGN at 3.5<z<7 identified by the deep (~8 hr) G395M THRILS survey. We investigate the photometrically selected EELGs in a color-color plot designed for ``Little Red Dot'' selection and demonstrate that it effectively removes AGN with non-extreme lines from the sample. EELGs with and without broad lines show similar optical line ratios. We compare emission-line morphology to EWs and continuum morphologies and find that [OIII] morphology is more compact at higher EW. ~10% of photometrically selected EELGs have broad Balmer lines, jumping to 35% in deep spectroscopy which indicates a significant fraction of photometrically selected EELGs may host AGN. However, many AGN selected as EELGs have incorrectly high photometric EWs. For sources with extreme emission-line EWs that pass our photometric criteria and host an AGN, we find that the narrow H alpha component dominates over the broad, especially in the highest-EW sources. This implies that even when an AGN is present, it does not dominate the extreme emission.

Short digest

JWST/NIRSpec spectra from CEERS, CAPERS, RUBIES, and deep THRILS observations vet a photometric sample of extreme emission-line galaxies (EWobs >5000 Å in Hα or Hβ+[O III]) in CEERS and test a Little Red Dot (LRD) color–color cut. Six new broad-line AGN are found at 3.5<z<7 in ~8 hr G395M THRILS data, yet EELGs with and without broad lines have similar optical line ratios and [O III] emission becomes more compact with increasing EW. Photometric and spectroscopic EWs generally agree within a factor of ~3, but AGN often have inflated photometric EWs from blue UV/red optical continua, with only ~10% (rising to ~35% with deeper spectra) showing broad Balmer lines. When AGN are present the narrow Hα dominates—especially at the largest EWs—implying the extreme emission is powered primarily by young star formation, not the AGN.

Key figures to inspect

• Figure 1: Inspect how the EW>5000 Å selection maps to spectroscopic EWs; note which photometric EELGs fall below threshold after spectroscopy and that AGN are over-represented among the discrepant high-EW photometric outliers.
• Figure 2: Check zphot–zspec systematics and the specific confusion tracks (Hα↔[O III], Lyman vs Balmer breaks); then compare EWphot vs EWspect to see the ~×3 agreement and identify BL AGN as EW-inflated outliers.
• Figure 3: Contrast shallow RUBIES with deep THRILS for the same source; the deep exposure reveals broad Balmer plus He I λ5876, λ7065, illustrating why the BL-AGN fraction rises to ~35% with deeper/grating data.
• Figure 4: Read the broad-component fraction vs EW; at the highest Hα or Hβ+[O III] EWs the narrow component dominates, directly supporting a star-formation origin for the extreme lines even in AGN hosts.