JWST has revealed a population of extreme emission line galaxies at high redshift — galaxies with spectral emission lines so strong that they dominate the broadband photometry. Equivalent widths of hydrogen-alpha exceeding 1000 angstroms, meaning the emission line contributes more light than the underlying stellar continuum across a broad filter. These galaxies light up in rest-frame optical surveys like flares.
The natural suspicion is that they're powered by active galactic nuclei — supermassive black holes accreting gas and producing intense ionizing radiation. AGN produce the hardest photons, the broadest lines, the most extreme equivalent widths. When you see extreme emission, you look for a black hole.
Davis, Brooks, Trump, and collaborators (arXiv 2602.23310, February 2026) use JWST NIRSpec spectroscopy from the CEERS survey to test this suspicion directly. They identify six new broad-line AGN at redshifts 3.5 to 7 among the extreme emission line galaxy population. About 35% of their photometrically selected sources host detectable AGN. The AGN are there.
But they're not driving the extreme emission. Even in the galaxies that host AGN, the narrow hydrogen-alpha component — the emission from gas ionized by stars, not the black hole — dominates over the broad component from the AGN. The extreme equivalent widths come from star formation, not accretion. The galaxies are building stars at a pace intense enough to produce the extreme line emission without needing a black hole to help.
The photometric selection method — choosing galaxies by their extreme equivalent widths — effectively filters out AGN-dominated systems. Galaxies where the AGN dominates the emission have lower equivalent widths (the AGN continuum dilutes the lines), so they fall below the selection threshold. The selection preferentially picks up systems where star formation is the dominant ionizing source. The extreme emission is a signature of extreme star formation efficiency, not extreme accretion.
The result shifts the interpretation of the high-redshift extreme emission line galaxy population. These are not disguised AGN. They are genuine starbursts — galaxies converting gas to stars at rates that produce ionizing photon budgets comparable to what AGN produce, but distributed across thousands of young star-forming regions rather than concentrated in a single accretion disk. The furnace is distributed, not central.