Little red dots as obscured little blue dots: Relative abundances, luminosities, and black hole masses
Year: 2026
Type: article
Abstract: We tested whether the so-called 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 ultraviolet (UV) luminosity function (LF) of broad-line active galactic nuclei (BLAGNs) at z ≳ 4 as the parent distribution, we forward-modeled the effects of the 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 and a dusty circumnuclear cloud population reproduces the LRD LF over the luminosity range currently constrained by the James Webb Space Telescope. The predicted LRD-to-BLAGN ratio is strongly luminosity dependent, rising from ≃3% at M 1500 = −21 mag 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 27% at M 4500 = −20 mag and 39% at M 6500 = −21. The fiducial solution adopted a baseline host extinction A V host = 0.5 mag, a characteristic per-cloud nuclear extinction ⟨ A V c ⟩ = 3.0 mag, and a gray R V = 5.5 attenuation curve, and it yields a mean dust covering factor ⟨ C dust ⟩≃0.23. These results can 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. In our model, the UV continuum is dominated by the active nucleus; LRDs and LBDs are drawn from the same observed BLAGN UV LF, but attenuation makes UV-selected LRDs intrinsically brighter and their inferred black holes more massive at fixed observed M 1500 .
Source: Astronomy and Astrophysics
Authors: Piero Madau, Roberto Maiolino
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
OpenAlex ID: W7160548849