Chasing the light: Shadowing, collimation, and the super-Eddington growth of infant black holes in JWST broad-line AGNs

TL;DR

This paper models super-Eddington accretion in high-redshift AGNs using thick disk geometries to explain their emission properties and rapid black hole growth.

Contribution

It introduces a geometrically thick, non-advective disk model to explain the ionization structure and emission lines of JWST-observed high-z BLAGNs.

Findings

Predicted blue optical-UV continuum slopes and collimated radiation fields.

Explained faint high-ionization lines despite strong Balmer emission.

Matched observed Balmer equivalent widths with typical BLR covering factors.

Abstract

Observations with the James Webb Space Telescope (JWST) have uncovered a substantial population of high-redshift broad-line active galactic nuclei (BLAGNs) characterized by moderate luminosities, weak X-ray emissions, and faint high-ionization lines. We propose that a subset of these BLAGNs, the so-called "little blue dots" (LBDs) are accreting at super-Eddington rates and use geometrically thick, non-advective disk models to investigate photon scattering and shadowing within the polar funnel. Our models predict extremely blue optical-UV continuum slopes and highly collimated radiation fields where isotropic-equivalent luminosities exceed the Eddington limit in the polar direction, while shadowing suppresses emission at higher inclinations. This "searchlight" configuration naturally generates a stratified ionization structure: coronal and high-excitation narrow lines are produced along the symmetry axis, while the equatorial broad-line region (BLR) remains shielded from the hardest ionizing photons. We show that the anisotropic illumination of the BLR explains the observed faintness of high-ionization lines despite strong Balmer emission. For M_BH=10^{7.5}-10^8 Msun black holes accreting at Eddington ratios ~10, standard BLR conditions predict HeII 4686/Hbeta in the range of 0.08-0.28. Notably, because inherently blue disk spectra provide a much higher ratio of ionizing to optical photons than standard quasar composites, the observed large Balmer equivalent widths are matched with typical BLR covering factors without invoking enshrouded geometries. Taken together, these findings support the view that super-Eddington accretion flows, shaped by thick disk geometries, may naturally account for the ionizing SED and emission line diagnostics of high-$z$ LBDs, while offering a plausible pathway to rapid black hole growth at cosmic dawn.