Little red dots as obscured little blue dots: relative abundances, luminosities, and black-hole masses

Piero Madau, Roberto Maiolino

First listed 2026-05-06 | Last updated 2026-05-06

Abstract

We test whether ``little red dots'' (LRDs) are the dust-reddened, high-inclination counterparts of bluer compact broad-line active galactic nuclei, here referred to as ``little blue dots'' (LBDs), by modeling their relative number densities and luminosities. Using the observed UV luminosity function (LF) of broad-line active galactic nuclei (BLAGNs) at z>4 as the parent distribution, we forward-model the effects of accretion rate, anisotropic emission, orientation, and dust obscuration within our super-Eddington unification framework. We show that a model with a geometrically thick accretion flow, an equatorially concentrated broad-line region, and a dusty circumnuclear cloud population reproduces the LRD LF over the luminosity range currently constrained by JWST. The predicted LRD/BLAGN fraction is strongly luminosity dependent, rising from 3% at M_1500=-21 to a peak value of 20% near M_1500=-19. The model also predicts a larger apparent LRD fraction at rest-frame optical wavelengths, reaching 26% at M_4500=-20 mag and 35% at M_6500=-21. The best-fitting solutions imply a characteristic per-cloud extinction <A_V>=2.8^{+0.0}_{-0.4} mag and a mean dust covering factor <C_dust>= 0.23^{+0.27}_{-0.00} at 68% confidence, with the asymmetric uncertainties reflecting the degeneracy between cloud extinction and covering factor. These results may support an orientation-based unification of little dots and identify the LRD LF as a key demographic test of rapid accretion onto infant black holes at cosmic dawn. Within this same framework, UV-selected LRDs are predicted to host systematically more massive black holes than unobscured LBDs, not because they represent a distinct parent population, but because dust attenuation preferentially removes lower-mass obscured systems from the observed UV sample.

Short digest

Madau and Maiolino test an orientation-based unification in which little red dots are simply dust-reddened, high-inclination versions of compact broad-line little blue dots drawn from the same z>4 BLAGN parent luminosity function. Their forward model combines super-Eddington anisotropic emission, an equatorially concentrated broad-line region, and clumpy circumnuclear dust, and it reproduces the observed UV LRD luminosity function over the JWST-constrained range. The inferred LRD fraction is strongly luminosity dependent, rising from about 3% at M_1500=-21 to a peak near 20% at M_1500=-19, with even larger apparent fractions predicted in the rest-frame optical, while the best fits favor per-cloud extinction around A_V≈2.8 and a modest mean dust covering factor of about 0.23. A notable consequence is that UV-selected LRDs should appear to host more massive black holes than unobscured LBDs, not because they are a separate population, but because dust preferentially removes lower-mass obscured systems from UV-selected samples.

Key figures to inspect

• Inspect Figure 1 first: it is the core demographic test, showing how the modeled LRD UV luminosity function sits below and steepens relative to the parent BLAGN LF, and how the apparent LRD/BLAGN fraction changes with observed M_1500.
• Figure 2 is the key caveat figure because it visualizes the degeneracy between per-cloud extinction and mean dust covering factor; it shows that similar LRD counts can be produced by either stronger clouds with less covering or weaker clouds with more covering.
• Figure 3 is the paper's payoff for black-hole demographics: compare the mass distributions of unobscured sightlines, all dust-intercepted systems, and the photometric LRD subset to see why UV-selected LRDs are biased toward higher inferred black-hole masses.
• Within Figure 1, pay particular attention to the lower panel confidence band around the LRD fraction, since that directly encodes the paper's headline prediction that the obscured fraction is luminosity dependent rather than a simple geometric constant.
• If the full paper includes any schematic of the thick-flow plus equatorial dust geometry, inspect it alongside Figures 1-2 to connect the phenomenology—anisotropic continuum, self-shadowing, and clumpy interception—to the fitted luminosity-function trends.