Little Red Dots as Obscured Little Blue Dots: A Super-Eddington Unification Model

Piero Madau, Roberto Maiolino

First listed 2026-02-25 | Last updated 2026-02-25

Abstract

We test whether "Little Red Dots" (LRDs) are dust-reddened, high-inclination counterparts of compact, blue broad-line AGNs ("Little Blue Dots", LBDs) powered by super-Eddington accretion. We model the central engine as a geometrically thick, radiation-pressure supported accretion flow whose funnel yields strongly anisotropic, intrinsically blue ionizing continua, coupled to an equatorially concentrated BLR and a dusty screen with modest covering factor. Using inclination-dependent SEDs as input to Cloudy, we show that the extreme broad Halpha equivalent widths (EWs) of JWST LRDs are reproduced with global BLR covering factors of only 15%, consistent with standard Type 1 AGNs. Large Balmer EWs arise because self-shadowing suppresses the high-inclination optical continuum while the BLR is illuminated by an EUV-rich SED. Weak high-ionization lines follow from orientation-dependent suppression of the XUV/soft X-ray continuum toward equatorial directions, without requiring a fully enclosing gaseous "cocoon". With a gray attenuation law of AV = 2.8 along LRD-selected sightlines, the fiducial model matches the V-shaped UV-optical continua and large Balmer decrements; strong Balmer breaks occur only for the most obscured views. A compact equatorial dust component tied to the BLR and normalized by energy conservation intercepts and reradiates only a small fraction of Lbol, producing a modest hot-dust bump and far-IR/sub-mm emission consistent with current limits and implying small dust masses. The model unifies LRD and LBD observables via orientation, predicting correlated trends in Halpha EW, Balmer decrement, Balmer break, high-ionization line strengths, and IR emission.

Short digest

Tests an orientation-based unification where Little Red Dots are dust-reddened, high-inclination counterparts of Little Blue Dots powered by super-Eddington accretion, using a thick, radiation-pressure supported flow with an anisotropic blue SED, an equatorial BLR, and a modest-covering dusty screen. Inclination-dependent SEDs fed to Cloudy reproduce the extreme broad Hα EWs with only ~15% global BLR covering factor and, with a gray AV=2.8 along LRD sightlines, match the V-shaped UV–optical continua, large Balmer decrements, and strong Balmer breaks only in the most obscured views. Weak high-ionization lines arise from orientation-dependent suppression of the XUV/soft X-ray continuum toward equatorial directions, removing the need for a fully enclosing cocoon; a compact equatorial dust component reradiates only a small fraction of Lbol (modest hot-dust bump; far-IR/sub-mm limits imply small dust masses). The model unifies LRD and LBD observables via orientation and predicts linked trends among Hα EW, Balmer decrement/break, high-ionization line strength, and IR output—offering concrete, testable diagnostics for early black-hole growth.

Key figures to inspect

• Figure 1: Check how ionizing photon rates above H I/He I/He II/N IV/Ne IV thresholds drop by 2–3 dex from face-on to edge-on, quantifying the self-shadowing that weakens high-ionization lines for LRD-like inclinations.
• Figure 2: Inspect the 3D thick-torus and funnel geometry to see how an equatorial BLR plus a circumnuclear dusty screen intercepts polar-beamed radiation, linking inclination to continuum dimming and BLR illumination.
• Figure 3: Read the Hα EW PDFs versus ionization parameter to see that matching the observed median requires only CF≈0.15–0.19 and that the high-EW tail maps to i≳70°, consistent with stacked LRD measurements.
• Figure 4: Compare Hα and Hβ EW versus inclination to verify that large Balmer EWs favor i≈60°–80°, aligning the LRD selection with geometry and setting expectations for Balmer decrement/break trends.