Ruling out dominant electron scattering in Little Red Dots' Rosetta Stone using multiple hydrogen lines

Matilde Brazzini, Francesco D'Eugenio, Roberto Maiolino, Ignas Juodžbalis, Xihan Ji, Jan Scholtz

First listed 2025-07-11 | Last updated 2025-07-11

Abstract

The majority of Little Red Dots (LRDs) hosting Active Galactic Nuclei (AGN) exhibits broad H$α$ emission, which recent studies propose originates from scattering off free electrons within an ionized and dense medium embedding the Broad Line Region (BLR), rather than directly from the BLR itself. This model suggests that the observed broad lines may be intrinsically narrower than observed, which would lead to black hole masses that are up to two orders of magnitude smaller than what inferred when assuming that the whole broad line comes from the BLR. To test this model, we present a joint analysis of multiple hydrogen recombination lines in the ''Rosetta Stone''AGN, the brightest known LRD at $z$=2.26. We show that H$α$, H$β$ and Pa$β$ have different spectral profiles, which is inconsistent with the predictions of the simple electron scattering scenario. Additionally, we test a variety of exponential models and show that none of them can simultaneously reproduce all three line profiles with physically plausible parameters. The inadequacy of these models for the Rosetta Stone implies that the scenario of electron scattering by an ionized medium surrounding the BLR is not universally applicable to LRDs and AGN, and therefore provides a counterexample to the claim of a universal and systematic overestimation of black hole masses.

Short digest

Targets the LRD “Rosetta Stone” GN-28074 (z=2.26) and tests whether broad Balmer/Paβ wings are dominated by wavelength-independent electron scattering. Using JWST/NIRSpec MSA medium-resolution data (∼1.7 hr), the authors compare Hα, Hβ, and Paβ profiles in velocity space and find mismatched wing shapes, contrary to the electron-scattering prediction of identical profiles. Joint fits with Gaussian BLR components plus exponential scattering wings cannot simultaneously match all three lines with physically plausible parameters. Result: dominant electron scattering around the BLR is ruled out for GN-28074, arguing against a universal systematic BH-mass overestimation in LRDs.

Key figures to inspect

• Velocity-space comparison of Hα, Hβ, and Paβ on a logarithmic flux scale, normalized on the red wing (≈+500 km s⁻¹): inspect the unequal wing slopes/curvatures on blue vs red sides that violate wavelength-independent scattering.
• Per-line decomposition panels (narrow + [O III]-tied outflow + broad BLR Gaussian + exponential-scattered component): check that best-fit exponential widths and scattered fractions differ across Hα/Hβ/Paβ, highlighting the model inconsistency.
• [O III] λλ4959,5007 kinematic anchor fit: verify the fixed outflow velocity/dispersion adopted for the Balmer fits and how subtracting this component affects the hydrogen-line wings.
• Joint-fit residuals across the three hydrogen lines: look for coherent, line-dependent residuals in the wings that demonstrate the failure of a single scattering kernel to reproduce all profiles.
• Balmer absorption modeling for Hα and Hβ (plus He I): examine the blueshift, covering factor, and depth to see how absorption alters the blue wings yet still cannot reconcile the inter-line shape differences.