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

TL;DR

The paper tests whether electron scattering in an ionized envelope around the BLR can explain the broad hydrogen lines in Little Red Dots. Using GN-28074 (Rosetta Stone) at z=2.26, they perform a joint Bayesian fit to H-alpha, H-beta, and Pa-beta with a line model comprising a BLR Gaussian, an exponential scattered wing, and an outflow component anchored to [O III], plus Balmer absorption. They find the lines exhibit different wing shapes and no single exponential width and scattered fraction can reproduce all three lines under plausible physics, casting doubt on the universal electron-scattering scenario. Pa-beta width is not suppressed as Balmer scattering would predict, and the data favor alternative explanations for broad wings, such as BLR dynamics or winds. Thus GN-28074 serves as a counterexample to the claim of universal overestimation of BH masses due to scattering, highlighting the importance of multi-line diagnostics when inferring SMBH masses.

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.

Figures (5)

• Figure 1: Line profile comparison in velocity space and on logarithmic scale for H$\alpha$, H$\beta$ and Pa$\beta$. Their fluxes are normalised to the red 500 $\mathrm{km\,s^{-1}}$. Spectral errors are reported in lighter colours.
• Figure 2: Best-fit models for H$\beta$ (left), H$\alpha$ (centre) and Pa$\beta$ (right). Individual model components are reported in different colours; each line is decomposed in three Gaussians (narrow, outflow and broad unscattered components) and the exponential convolution representing the broad scattered component. For the H$\beta$ and H$\alpha$, the absorber is also modelled. The three inset panels report the line profiles in logarithmic scale, with the best-fit model in red and the exponential component in blue.
• Figure 3: Posterior distributions of FWHM$_\text{broad}$, $W$ and $f_\text{scatt}$ for H$\alpha$ (teal), H$\beta$ (orange), and Pa$\beta$ (maroon).
• Figure 4: Fit of [O iii]$\lambda\lambda 4959,5007$ doublet. Each line is modelled with a narrow and a double-Gaussian outflow components. In the lower panel, we compare the residuals of the fit with the single-Gaussian outflow model, to highlight the inadequacy of the latter in correctly reproducing oxygen emission.
• Figure C1: Posterior distributions of FWHM$_{\text{broad}}$, $W$, and $f_\text{scatt}$ from models 2 and 3. In model 2 the three line widths are bound and the exponential parameters are free to vary, while in model 3 it is the opposite. Common parameters are displayed in green, while single-line parameters are displayed in teal for H$\alpha$, gold for H$\beta$, and maroon for Pa$\beta$.