Wings of little dots: Exponential broad lines from a stratified broad-line region

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Abstract
We investigate the origin of the broad exponential wings seen in the H α profiles of many JWST-discovered little red dots (LRDs) and little blue dots (LBDs). Recent studies show that exponential broad-line profiles are not unique to LRDs, are common in LBDs too, and need not imply that electron scattering is the dominant broadening mechanism in every source. We model the broad Balmer emission with a virialized, radially stratified broad-line region (BLR), motivated by our unification picture, in which LRDs are dust-reddened, high-inclination counterparts of compact blue broad-line active galactic nuclei (AGNs). In this framework, the observed profile is the luminosity-weighted superposition of clouds spanning a range of radii, and hence virial velocities. We show that a stratified BLR reproduces the exponential-like wings observed in three representative LRDs and one LBD, without requiring electron scattering as the dominant broadening mechanism. For the LBD GS-3073, the same BLR – calibrated only to H α – predicts, parameter-free, that He II λ4686 and H β should be broader than H α , consistent with observations and offering an independent test of BLR ionization stratification. Our results support a picture in which wings and cores encode different physics: the wings arise primarily from virial BLR stratification, while the cores retain imprints of absorption and radiative transfer in dense gas. The fits further suggest that the cloud radial distribution peaks near the dust sublimation radius, with the exponential wings shaped by the inner, line-emitting BLR shells whose 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.

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