Chasing the Light: Shadowing, Collimation, and the Super-Eddington Growth of Infant Black Holes in JWST BLAGNs

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 accretors at Eddington ratio ~10, standard BLR conditions predict HeII lambda 4686/Hbeta in the range 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.

Figures (9)

• Figure 1: Meridional cross sections (over one quadrant) for the supercritical thick disks described by Models A (cyan curve), B (magenta curve), and C (green curve). As $r_{\rm in}$ decreases, $r_{\rm out}$ increases and so does the ratio $L_{\rm rad}/L_{\rm Edd}$, while the efficiency of mass to energy conversion becomes progressively smaller. Smaller values of $r_{\rm in}$ also imply steeper and deeper funnels, where pressure gradients are balanced by centrifugal forces rather than by gravity and luminosities can exceed the Eddington limit. The square symbols mark the location on the surface inside which 90% of the disk luminosity $L_{\rm rad}$ is actually emitted. The inset shows the Keplerian specific angular momentum distribution for the adopted pseudo-Newtonian potential (black line) and the three angular momentum distributions corresponding to our supercritical disks (Models A, B, and C).
• Figure 2: The 3D toroidal geometry (including shadowing) of a supercritical accretion disk (Model B). The structure is shown at inclination angles $i=45^\circ$ (top) and $i=15^\circ$ (bottom), with $r$ and $z$ expressed in units of $r_S$. The shadow boundary depends on both radius and azimuth (see Appendix A), and the resulting geometry produces pronounced anisotropy that exceeds the standard inclination–induced anisotropy of a thin accretion disk. The thick funnel extends out to $\sim 300\,r_S$ where it is matched to an outer geometrically thin disk shown extending to $r=500\,r_S$ to illustrate the geometric transition. The color coding represents the bolometric surface brightness of the accretion disk, where brighter (yellow) colors indicate regions of higher intensity and darker (blue) colors indicate lower intensity.
• Figure 3: Equivalent isotropic luminosity $L^i$ (in units of $L_{\rm Edd}$) for supercritical disk Models A (turquoise line), B (magenta line), and C (green line) as measured by an observer at different inclination angles $i$. The labels along the curves mark the true isotropic luminosities (in units of Eddington) radiated by the thick disks. The square symbols mark the "half-power beam width" -- the inclination angle $i_{1/2}$ at which the observed luminosity falls to 50% of the peak value. In the wider opening angle case, there is little beam-broadening by scattered radiation.
• Figure 4: Thick disk non-blackbody spectra for Model B. The calculations assume a black hole mass of ${M_{\rm BH}}=10^{7.5}\,\,{M_\odot}$, a scattering-dominated (constant density) atmosphere with gas to total pressure ${\cal P}=10^{-4}$, and include the self-illumination effect of the funnel walls.
• Figure 5: Inclination dependence of the continuum slopes $\beta$, defined by fitting the emergent spectrum with a power law $f_\lambda(i)\propto \lambda^\beta$ over six rest-frame wavelength windows: softX (50--200 Å), XUV (200--600 Å), EUV (600--900 Å), FUV (1200--1800 Å), UV (2000--3000 Å), and optical (4500--6500 Å), as indicated in the legend. The calculations assume a fiducial super-Eddington accretor with $M_{\rm BH}=10^{7.5}\,M_\odot$ and $\dot m=12.4$ (Model B). At low-to-intermediate inclinations ($i\lesssim35^\circ$) the slopes are nearly constant, while toward edge-on views the ionizing continuum softens: $\beta_{\rm softX}$, $\beta_{\rm XUV}$, and $\beta_{\rm EUV}$ increase markedly, whereas the FUV changes only modestly and the UV and optical slopes remain nearly unchanged.