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Arxiv: Challenging the AGN scenario for JWST/NIRSpec broad Hα emitters/Little Red Dots in light of non-detection of NIRCam photometric variability and X-ray Published: 7/5/2024 9:00:03 PM Updated: 7/5/2024 9:00:03 PM

Paper abstract: JWST has uncovered a substantial population of high-z (z \gtrsim 4)galaxies exhibiting broad H\alpha emission line with a Full Width at HalfMaximum exceeding 1,000~km~s^{-1}. This population includes a subset knownas 'Little Red Dots', characterized by their compact morphology and extremelyred rest-frame optical colors. If all of these broad H\alpha emitters wereattributed to type 1-1.9 Active Galactic Nuclei (AGNs), it would imply asignificantly higher number density of low-luminosity AGNs than extrapolatedfrom that of more luminous AGNs. Here, we have examined the rest-frameultraviolet (UV)-optical flux variability of five JWST broad H\alpha emittersusing multi-epoch, multi-band JWST/NIRCam imaging data. The rest-frame temporalsampling interval of the NIRCam data (~ 400-500~days/(1+z)) iscomparable to typical variability timescales of AGNs with black hole (BH)masses of M_{\text{BH}} ~ 10^{7}~M_{\odot}; thus, the flux variationsshould be detectable if AGNs were present. However, no measurable fluxvariation over the rest-frame wavelength range of \lambda_{\text{rest}} ~1,500-9,000\AA\ has been detected, placing stringent upper limits on thevariability amplitudes. This result, combined with the X-ray faintnessconfirmed by the ultra-deep {\it Chandra} data, indicates that, under the AGNscenario, we need to postulate peculiar Compton-thick broad-line AGNs witheither (a) an intrinsically non-variable AGN disk continuum, (b) a hostgalaxy-dominated continuum, or (c) scattering-dominated AGN emission.Alternatively, (d) they could be non-AGNs where the broad-line emissionoriginates from unusually fast and dense/low-metallicity star-formation-drivenoutflows or inelastic Raman scattering of stellar UV continua by neutralhydrogen atoms.