Little Red and Blue Dots: simply stratified Broad Line Regions

J. Scholtz, F. D'Eugenio, R. Maiolino, M. Brazzini, H. Übler, X. Ji, M. Perna, F. Sun, G. Brocchi, S. Carniani, G. Cresci, L. R. Ivey, I. Juodžbalis, A. Marconi, G. Mazzolari, G. Risaliti, B. Trefoloni

First listed 2026-03-23 | Last updated 2026-03-24

Abstract

It has been claimed that a fraction of the so-called Little Red Dots (LRDs) are characterised by exponential broad line profiles, which have been ascribed to broadening from electron scattering by an ionised cocoon. In this work, we investigate the H$α$ broad line profiles of 32 AGN, including Little Red Dots (LRDs), Little Blue Dots (LBDs), and X-ray detected sources, using high SNR and resolution spectroscopy. We find that while single Gaussian models are statistically rejected, the exponential model is not universally preferred. Lorentzian and multi-Gaussian profiles provide equally good or superior fits for the majority of the sample, with no statistical preference for exponential profiles in $\sim$60% of cases across all AGN subtypes. There are indications that exponential profiles are preferred more frequently among LBDs, indicating that exponential profiles are not a prerogative of LRDs, which actually seem to more often favour Lorentzian profiles. Furthermore, we demonstrate that exponential wings can emerge naturally from the stratification of BLR clouds in virial motion, without invoking any scattering process. More generally, we also show that stacking multiple broad lines (either from multiple objects, as done in previous works, or from different BLR components within the same object) generally yields an exponential profile, even if none of the individual profiles are exponential. Explaining the exponential profiles in terms of BLR stratification solves various observational tensions with the electron scattering interpretation. While electron scattering may play a role, there is no evidence that it dominates the line profiles and that it significantly affects the inferred black hole masses.

Short digest

Using high-S/N, R≈1000–2700 Hα spectra of 32 type‑1 AGN spanning Little Red Dots, Little Blue Dots, and X‑ray sources, the authors compare broad-line profile families. Single Gaussians are ruled out, while exponential shapes are not uniquely preferred; Lorentzian or multi‑Gaussian fits match or outperform them for most objects, with no statistical preference in ~60% of cases and a tendency for LRDs to favor Lorentzians and LBDs to show exponentials. They demonstrate that exponential wings arise naturally from a stratified, virial BLR and that stacking multiple broad lines (across objects or internal components) generically produces an exponential profile. This removes the need for a dominant electron‑scattering cocoon and implies standard virial black‑hole mass estimates remain broadly valid, though scattering may contribute at some level.

Key figures to inspect

• Exemplar Hα profile fits for a representative LRD and LBD, showing exponential vs Lorentzian vs multi‑Gaussian models with residuals—use this to see where each model wins (core vs wings) and why single Gaussians fail.
• Bar or fractional breakdown of best‑fit profile types across the 32 objects, split by subtype (LRD, LBD, X‑ray)—inspect the higher exponential fraction in LBDs and the Lorentzian tendency in LRDs, and note the ~60% with no unique statistical preference.
• Stacking experiment figure—compare individual-object Hα profiles to the stacked result to see how an exponential emerges even when no single source is exponential, illustrating why stacking can bias interpretation.
• BLR stratification toy model or simulation—radial/velocity stratification of virial clouds and the resulting synthetic line profile; check that exponential wings appear without invoking electron scattering.
• Impact on MBH—comparison of FWHM/line‑widths and inferred black‑hole masses under different profile assumptions vs an electron‑scattering interpretation, showing minimal change to virial MBH estimates.