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

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. This population consists of a well-known subset called Little Red Dots (LRDs) and the remaining non-LRDs. 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 three LRD and two non-LRD broad H$\alpha$ 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 ${\lambda }_{\text{rest}}\sim 1,500-9,000$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.

35 pages, 13 figures, 6 tables, accepted for publication in the Astrophysical Journal 2025 October 7

References in corpus (31)

• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• JADES. The diverse population of infant Black Holes at 4<z<11: merging, tiny, poor, but mighty
• GA-NIFS: A massive black hole in a low-metallicity AGN at $z\sim 5.55$ revealed by JWST/NIRSpec IFS
• The GLASS James Webb Space Telescope Early Release Science Program. I. Survey Design and Release Plans
• Radiative Transfer Modeling of Three-Dimensional Clumpy AGN Tori and its Application to NGC 1068
• JWST CEERS & JADES Active Galaxies at z = 4-7 Violate the Local $M_{\bullet }-M_{\star }$ Relation at $>3\sigma$: Implications for Low-Mass Black Holes and Seeding Models
• A characteristic optical variability timescale in astrophysical accretion disks
• RUBIES: Evolved Stellar Populations with Extended Formation Histories at $z\sim 7-8$ in Candidate Massive Galaxies Identified with JWST/NIRSpec
• A Candidate of a Least-Massive Black Hole at the First 1.1 Billion Years of the Universe
• UNCOVERing the extended strong lensing structures of Abell 2744 with the deepest JWST imaging
• Limitations on the recovery of the true AGN variability parameters using Damped Random Walk modeling
• Intensive disc-reverberation mapping of Fairall 9: 1st year of Swift & LCO monitoring
• Improving Damped Random Walk parameters for SDSS Stripe 82 Quasars with Pan-STARRS1
• A dormant, overmassive black hole in the early Universe
• Deciphering the JWST spectrum of a 'little red dot' at $z\sim 4.53$: An obscured AGN and its star-forming host
• JADES -- The Rosetta Stone of JWST-discovered AGN: deciphering the intriguing nature of early AGN
• Optical Variability of Narrow and Broad line Seyfert 1 galaxies
• Supernovae in Low-Redshift Galaxy Clusters: Observations by the Wise Observatory Optical Transient Search (WOOTS)
• Do Some AGN Lack X-ray Emission?
• The GLASS-JWST Early Release Science Program. II. Stage I release of NIRCam imaging and catalogs in the Abell 2744 region
• Exploring the AGN Fraction of a Sample of JWST's Little Red Dots at $5<z<8$: Overmassive Black Holes Are Strongly Favored
• Constraints on the temperature inhomogeneity in quasar accretion discs from the ultraviolet-optical spectral variability
• Dwarf AGNs from Variability for the Origins of Seeds (DAVOS): Intermediate-mass black hole demographics from optical synoptic surveys
• Properties of AGN Multiband Optical Variability in the HSC SSP Transient Survey
• A Long-Duration Luminous Type IIn Supernova KISS15s: Strong Recombination Lines from the Inhomogeneous Ejecta-CSM Interaction Region and Hot Dust Emission from Newly Formed Dust
• Raman mapping of photodissociation regions
• Rayleigh and Raman scattering cross-sections and phase matrices of the ground-state hydrogen atom, and their astrophysical implications
• Constraining the AGN luminosity function from JWST with the X-ray Background
• Monitoring broad emission-line components in spectra of the two low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840
• Differencing and Coadding JWST Images with Matched Point Spread Function

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• Impact of Resonance, Raman, and Thomson Scattering on Hydrogen Line Formation in Little Red Dots
• Spectral Uniformity of Little Red Dots: A Natural Outcome of Coevolving Seed Black Holes and Nascent Starbursts
• Primordial Black-Hole-Based Pathways to Little Red Dots
• Investigating the Growth of Little Red Dot Descendants at z<4 with the JWST
• The Cliff: A Metal-Poor Little Red Dot Hosting an Overmassive Black Hole at $z=3.55$
• NEXUS: A Search for Nuclear Variability with the First Two JWST NIRCam Epochs
• Discovery of low-redshift analogues to "Little Red Dots" in DESI: A later evolutionary stage of compact LRDs?
• Euclid: Discovery of 31 new quasars at $6.6<z<7.8$
• A Comprehensive JWST/NIRSpec Census of Broad-line Active Galactic Nuclei: Faint, Low-mass Accretors and a Population of Little Red Dots
• Mass and Spin Growth of Very Massive Stars in Star Clusters Potentially Associated with Little Red Dots