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

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.

Figures (8)

• Figure 1: Angle-dependent ionizing photon production rates, $Q(>E_{\rm th})$, for a super-Eddington accretion flow with $M_{\rm BH}=10^{7.5}\,M_\odot$ and $\dot m=32$ (Model A of Madau2025). The curves show the integrated photon rate (s$^{-1}$) above five ionization thresholds: H i (13.6 eV), He i (24.6 eV), He ii (54.4 eV), N iv (77.5 eV), and Ne iv (97.2 eV). The rates are strongly anisotropic, remaining near their peak values for $i\lesssim 25^\circ$ but declining steeply at high inclinations as self-shadowing obscures the hot inner funnel and the emergent spectrum becomes dominated by the cooler outer disk. From face-on to edge-on, the highest-threshold photon rates decrease by $\mathrel{\hbox{$\mathchar"218$} \hbox{$\mathchar"13E$}}\,$2--3 dex.
• Figure 2: Three-dimensional toroidal geometry of a super-Eddington BLAGN accreting at $\dot m=32$, viewed at an inclination angle of $i=60^\circ$. The spatial coordinates $x, y$, and $z$ are expressed in units of Schwarzschild radii $r_S$. The thick torus extends out to $\simeq 1000\,r_S$, where it is matched to an outer geometrically thin disk (only shown to $r=1500\,r_S$ to illustrate the geometric transition). Brighter (yellow) colors indicate higher surface brightness, while darker (blue) colors indicate lower values. The teal spheres provide a schematic (not-to-scale) visualization of BLR clouds; physically, the BLR lies at much larger radii ($\sim 10^5\,r_S$), and its outer portion beyond the sublimation radius is expected to be dusty. The outer brown structure schematically represents a dusty circumnuclear obscurer (a torus-like structure) surrounding the BLR.
• Figure 3: Probability density functions (PDFs) of the broad H$\alpha$ EW predicted by our fiducial super-Eddington accretor with $M_{\rm BH}=10^{7.5}\,M_\odot$ and $\dot m=32$. The distributions are derived by weighting the inclination-dependent EWs by the geometric probability $p(i)=\sin i$ ($0\le i\le \pi/2$), assuming random observer orientations. Different curves correspond to different ionization parameters $\log U$, as indicated in the legend. For each model, the global BLR covering factor $C_{\rm BLR}$ is scaled such that the probability-weighted median of the theoretical distribution matches the observed median EW of $570$ Å from the high-$z$ BLAGN sample of Maiolino2025 (vertical black line). The resulting required covering factors are listed in parentheses in the legend and are relatively modest ($C_{\rm BLR}=0.12$--0.19). The gray shaded region denotes the full range of EWs observed in the JADES sample ($200$--$1200$ Å). The high-EW tail above 1200 Å corresponds to highly inclined ($i\mathrel{\hbox{$\mathchar"218$} \hbox{$\mathchar"13E$}} 80^\circ$) observers. Also shown is the total (broad$+$narrow) EW recently measured from a median-stacked LRD spectrum by Sun2026 (magenta dashed line with shaded uncertainty band), which lies toward the high-EW (high-inclination) side of the distribution.
• Figure 4: Predicted inclination dependence of the Balmer EWs. The top and bottom panels show ${\rm EW}(\mathrm{H}\alpha)$ and ${\rm EW}(\mathrm{H}\beta)$, respectively, as functions of observer inclination for our fiducial super-Eddington model with $M_{\rm BH}=10^{7.5}\,M_\odot$ and $\dot m=32$. Curves correspond to different ionization parameters $\log U$ (legend), computed for a fixed BLR covering factor $C_{\rm BLR}=0.15$. The magenta dashed lines and shaded bands indicate the total (broad+narrow) EWs measured from the median-stacked LRD spectrum of Sun2026, ${\rm EW}(\mathrm{H}\alpha)=817.2^{+80.9}_{-82.7}\,\AA$ and ${\rm EW}(\mathrm{H}\beta)=109.8^{+6.8}_{-6.9}\,\AA$; at the PRISM resolution these values cannot in general be decomposed into broad and narrow components. In our model, matching the large Balmer EWs favours highly inclined sightlines, typically $i\gtrsim 65^\circ$--$70^\circ$ depending weakly on $\log U$.
• Figure 5: Predicted He ii$\lambda4686/\mathrm{H}\beta$ as a function of $\log U$ for super-Eddington SEDs of different $M_{\rm BH}$ and $\dot m$ (see legend). Unless otherwise noted, $n_{\rm H}=10^{10}\,{\rm cm^{-3}}$; the single alternative-density case is labeled in the legend. The dashed line shows the upper limit for the LRD Abell 2744--QSO1 from JWST spectroscopy Furtak2024Ji2025. All models assume $Z=0.1\,Z_\odot$ and $N_{\rm H}=10^{23}\,{\rm cm^{-2}}$.