James Webb Space Telescope Feed Post

Date: 7/9/2024

Harvard ADS: Challenging the AGN scenario for JWST/NIRSpec broad H\alpha emitters/Little Red Dots in light of non-detection of NIRCam photometric variability and X-ray

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 Half Maximum exceeding 1,000~km~s^{-1}. This population includes a subset known as 'Little Red Dots', characterized by their compact morphology and extremely red rest-frame optical colors. If all of these broad H\alpha 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 five JWST broad H\alpha emitters using multi-epoch, multi-band 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 (BH) masses of M_{\text{BH}} ~ 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 \lambda_{\text{rest}} ~ 1,500-9,000\AA\ has been detected, placing stringent upper limits on the variability amplitudes. This result, combined with the X-ray faintness confirmed by the ultra-deep {\it 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.