Little red dots as obscured little blue dots: relative abundances, luminosities, and black-hole masses
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 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 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 adopts 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 yields a mean dust covering factor < C_{dust} > ~0.23. This value is conditional on the adopted dust prescription, since lower per-cloud extinction can be compensated by a larger 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.
Related
- Little red dots as obscured little blue dots: A super-Eddington unification model
- Super-Earth masses and stellar abundances from NIRPS reveal tentative evidence for water-rich formation around M dwarfs
- UV spectra of little red dots: Broad-line N/C, gas recycling and black hole masses
- The obscured nucleus of NGC 3256,S. I. Anisotropic active galactic nucleus near-IR radiation field traced by CO v=1--0 with JWST NIRSpec
- AT2019aalc: An obscured tidal disruption event candidate in an active galactic nucleus revealed by its first flare?
BibTeX
@misc{madau2026little,
title = {{Little red dots as obscured little blue dots: relative abundances, luminosities, and black-hole masses}},
author = {Piero Madau and Roberto Maiolino},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2605.05074},
eprint = {2605.05074},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2605.05074},
}
RIS
TY - UNPB
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 - 2026/09/08
PB - arXiv
DO - 10.48550/arxiv.2605.05074
UR - https://arxiv.org/abs/2605.05074
AB - 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 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 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 adopts 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 yields a mean dust covering factor < C_{dust} > ~0.23. This value is conditional on the adopted dust prescription, since lower per-cloud extinction can be compensated by a larger 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.
ER -