Wings of little dots: Exponential broad lines from a stratified BLR

Piero Madau, Roberto Maiolino, Jan Scholtz, Francesco D'Eugenio

First listed 2026-04-05 | Last updated 2026-04-05

Abstract

We investigate the origin of the broad exponential wings observed in a significant fraction of the Halpha profiles of JWST-discovered little red dots (LRDs) and little blue dots (LBDs). Recent studies have shown that exponential broad-line profiles are not a prerogative of LRDs, are often also present in LBDs, and need not imply that electron scattering is the dominant broadening mechanism in every source. Motivated by our unification picture in which LRDs are the dust-reddened, high-inclination counterparts of compact blue broad-line AGNs, we model the broad Balmer emission with a virialized, radially stratified broad-line region (BLR). In this framework, the observed profile is the luminosity-weighted superposition of clouds spanning a range of radii and therefore a range of characteristic virial velocities. We show that such a stratified BLR can reproduce the extended exponential-like wings observed in three representative LRDs, without requiring electron scattering to be the primary origin of the broad wings. Our results support a picture in which the broad wings and the line cores encode different physics: the wings arise primarily from virial BLR stratification, whereas the cores retain additional imprints of absorption and radiative transfer in dense gas. The successful fits further suggest that the cloud radial distribution peaks near the dust sublimation radius, while the exponential wings are shaped by the line-emitting inner BLR shells where the higher virial velocities produce the high-velocity tails. This offers a simple physical explanation for the exponential wings of little dots, without invoking exotic new components or scenarios.

Short digest

This paper argues that the exponential broad Halpha wings seen in many LRDs do not require electron scattering as the primary explanation. The main result is that a virialized, radially stratified BLR naturally produces exponential-like wings when you sum emission from clouds across a range of radii and virial velocities. The paper matters because it separates the physics of the wings from the line cores: the wings mainly trace BLR stratification, while the cores can still retain the imprint of absorption and radiative-transfer effects in dense gas.

Key figures to inspect

• Figure 1 is the must-see plot: it shows the stratified-BLR fit to GN-68797, including the absorbed versus unabsorbed broad component, so this is the cleanest introduction to the paper's core argument.
• Figure 2 is the next figure to inspect: it repeats the same exercise for GS-13971 and shows that the exponential-wing explanation is not tuned to a single source.
• Figure 3 is where to test how general the picture is: the fit to GN-9771 shows that the same stratified-BLR interpretation can reproduce the broad-wing phenomenology across multiple representative LRDs.