Challenging the Active Galactic Nucleus Scenario for JWST/NIRSpec Little Red Dot and Non–Little Red Dot Broad Hα Emitters in Light of Nondetection of NIRCam Photometric Variability and X-Ray

JWST has uncovered a substantial population of high-z (z ≳ 4) galaxies exhibiting broad Hα emission lines with a full width at half-maximum exceeding 1000 km s−1. This population consists of a well-known subset called little red dots (LRDs) and the remaining non-LRDs. If all of these broad Hα emitters were attributed to type 1–1.9 active galactic nuclei (AGNs), it would imply a significantly higher number density of low-luminosity AGNs than extrapolated from that of more luminous AGNs. Here, we have examined the rest-frame ultraviolet (UV)-optical flux variability of three LRD and two non-LRD broad Hα emitters using multiepoch, multiband JWST/NIRCam imaging data. The rest-frame temporal sampling interval of the NIRCam data (∼400–500 days/(1 + z)) is comparable to typical variability timescales of AGNs with black hole masses of MBH ∼ 107 M⊙; thus, the flux variations should be detectable if AGNs were present. However, no measurable flux variation over the rest-frame wavelength range of λrest ∼ 1500–9000 Å has been detected, placing stringent upper limits on the variability amplitudes. This result, combined with the X-ray faintness confirmed by the ultradeep Chandra data, indicates that, under the AGN scenario, we need to postulate peculiar Compton-thick broad-line AGNs with either (a) an intrinsically nonvariable AGN disk continuum, (b) a host galaxy-dominated continuum, or (c) scattering-dominated AGN emission. Alternatively, (d) they could be non-AGNs where the broad-line emission originates from unusually fast and dense/low-metallicity star formation-driven outflows or inelastic Raman scattering of stellar UV continua by neutral hydrogen atoms.

Recommended citation: Kokubo (2025), ApJ, 995, 24
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