Wings of little dots: Exponential broad lines from a stratified broad-line region
5 affiliations
- 1Department of Astronomy & Astrophysics, University of California
- 2Dipartimento di Fisica “G. Occhialini,” Università degli Studi di Milano-Bicocca
- 3Cavendish Laboratory, University of Cambridge
- 4Department of Physics and Astronomy, University College London
- 5Kavli Institute for Cosmology, University of Cambridge
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.
Related
- Broad-band host measurements of little red dots and similar compact nuclei require source-conditioned polychromatic PSFs
- Lyα in Compact Broad-Line Emitters, Including Little Red Dots, Is More Closely Linked to the Host Galaxy than to the Broad-Line Nucleus
- Where did all the Little Red Dots go? The abundance of LRD analogues among objects with broad lines at z < 0.35
- UV spectra of little red dots: Broad-line N/C, gas recycling and black hole masses
- A Black Hole Star at Cosmic Noon: Extreme Balmer Break, Photospheric Continuum, and Broad Absorption by Thick Winds in a Little Red Dot at z = 1.7
BibTeX
@article{madau2026wings,
title = {{Wings of little dots: Exponential broad lines from a stratified broad-line region}},
author = {Piero Madau and R. Maiolino and Jan Scholtz and Francesco D'Eugenio},
journal = {Astronomy and Astrophysics},
year = {2026},
month = sep,
doi = {10.1051/0004-6361/202660251},
url = {https://doi.org/10.1051/0004-6361/202660251},
note = {CC BY},
}
RIS
TY - JOUR
TI - Wings of little dots: Exponential broad lines from a stratified broad-line region
AU - Piero Madau
AU - R. Maiolino
AU - Jan Scholtz
AU - Francesco D'Eugenio
PY - 2026
DA - 2026/09/01
JO - Astronomy and Astrophysics
DO - 10.1051/0004-6361/202660251
UR - https://doi.org/10.1051/0004-6361/202660251
AB - 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.
SN - 0004-6361
SN - 1432-0746
ER -