Little red dots as obscured little blue dots: Relative abundances, luminosities, and black hole masses
5 affiliations
- 1Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca
- 2Department of Astronomy & Astrophysics, University of California
- 3Kavli Institute for Cosmology, University of Cambridge
- 4Cavendish Laboratory, University of Cambridge
- 5Department of Physics and Astronomy, University College London
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 .
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BibTeX
@article{madau2026little,
title = {{Little red dots as obscured little blue dots: Relative abundances, luminosities, and black hole masses}},
author = {Piero Madau and Roberto Maiolino},
journal = {Astronomy \& Astrophysics},
year = {2026},
month = oct,
doi = {10.1051/0004-6361/202660815},
url = {https://www.aanda.org/10.1051/0004-6361/202660815},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Little red dots as obscured little blue dots: Relative abundances, luminosities, and black hole masses
AU - Piero Madau
AU - Roberto Maiolino
PY - 2026
DA - 2026/10/28
JO - Astronomy & Astrophysics
DO - 10.1051/0004-6361/202660815
UR - https://www.aanda.org/10.1051/0004-6361/202660815
AB - 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 .
SN - 0004-6361
SN - 1432-0746
ER -