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

ABSTRACT Most little red dots (LRDs) hosting active galactic nuclei (AGNs) show broad ${\rm H} \, \alpha$ emission, which recent studies ascribe to scattering off free electrons within an ionized medium embedding the broad-line region (BLR), rather than directly from the BLR itself. If correct, this model would imply intrinsically narrower broad line widths, leading to black hole masses that are up to two orders of magnitude smaller than what is inferred when assuming that the whole broad line emission comes from the BLR. To test this, we present a joint analysis of multiple hydrogen recombination lines in the ‘Rosetta Stone’ AGNs, the brightest known LRD at z = 2.26. We show that ${\rm H} \, \alpha$, ${\rm H} \, \beta$, and ${\rm Pa} \, \beta$ have different spectral profiles, which is inconsistent with the predictions of the simple 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 embedding the BLR is not universally applicable to LRDs and AGNs, and therefore provides a counterexample to the claim of a universal and systematic overestimation of black hole masses.