Challenging the AGN scenario for JWST/NIRSpec LRD and non-LRD broad H$α$ emitters in light of non-detection of NIRCam photometric variability and X-ray

TL;DR

The study tests whether JWST broad Hα emitters at z ≳ 4 are dominated by low-luminosity AGNs by searching for rest-frame UV–optical variability with multi-epoch NIRCam data, leveraging the expected AGN variability on 0.1 mag scales over months to years. It analyzes five objects (3 LRDs, 2 non-LRDs) in Abell 2744 and incorporates Chandra X-ray constraints. The non-detection of photometric variability and X-ray emission implies that the standard AGN scenario requires unusual configurations (e.g., Compton-thick BLRs, host-dominated continua, or scattering-dominated emission) or that the broad Hα originates from non-AGN processes such as extreme star-formation–driven outflows or Raman scattering. These results place stringent limits on the presence of low-mass AGNs in this population and motivate alternative explanations for the broad Hα emitters in the early universe, with JWST variability as a powerful diagnostic.

Abstract

JWST has uncovered a substantial population of high-$z$ ($z \gtrsim 4$) galaxies exhibiting broad H$α$ emission line with a Full Width at Half Maximum exceeding 1,000 km/s. This population consists of a well-known subset called Little Red Dots (LRDs) and the remaining non-LRDs. If all of these broad H$α$ 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 H$α$ emitters using multi-epoch, multi-band JWST/NIRCam imaging data. The rest-frame temporal sampling interval of the NIRCam data ($\sim 400-500$~days$/(1+z)$) is comparable to typical variability timescales of AGNs with black hole (BH) masses of $M_{\text{BH}} \sim 10^{7}~M_{\odot}$; thus, the flux variations should be detectable if AGNs were present. However, no measurable flux variation over the rest-frame wavelength range of $λ_{\text{rest}} \sim 1,500-9,000$Å has been detected, placing stringent upper limits on the variability amplitudes. This result, combined with the X-ray faintness confirmed by the ultra-deep Chandra data, indicates that, under the AGN scenario, we need to postulate peculiar Compton-thick broad-line AGNs with either ($a$) an intrinsically non-variable 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.

Figures (1)

• Figure 1: The NIRCam all-epoch mosaic images of the Abell 2744 field created from the multi-band multi-epoch data used in this study (see Section \ref{['sec:nircam_image_processing']} and Table \ref{['tbl:summary_data']}). Dashed lines in each panel denote the image footprints of the corresponding band, color-coded by the observation dates. The positions of the JWST broad H$\alpha$ emitters analyzed in this study are indicated by the $+$ and $\times$ symbols (har23's non-LRD broad H$\alpha$ emitters; and gre24's LRD broad H$\alpha$ emitters, respectively).