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

TL;DR

This study models the broad exponential wings in Halpha profiles of JWST-discovered little red and blue dots using a stratified broad-line region, challenging electron scattering as the primary broadening mechanism.

Contribution

It introduces a virialized, radially stratified BLR model to explain exponential wings in emission lines, avoiding the need for electron scattering.

Findings

Stratified BLR can reproduce exponential wings in LRDs.

Wings originate from inner BLR shells with high virial velocities.

Line cores are influenced by absorption and radiative transfer in dense gas.

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.