Broad-line Regions behind Haze: The Intrinsic Shape of the Br γ Line and Its Origin in a Type 1 Seyfert Galaxy

Abstract The broad-line region (BLR) of active galactic nuclei is an essential component, yet its small size keeps its origin, structure, and kinematics uncertain. Infrared interferometry with VLTI/GRAVITY is now resolving the BLR-scale emission, with data for NGC 3783 consistent with a rotating, geometrically thick configuration. However, the processes shaping the spectra remain poorly constrained, and the cloud models are tuned phenomenologically rather than derived from first-principle predictions. We address this by coupling three-dimensional radiation-hydrodynamic (RHD) simulations of gas around a supermassive black hole with radiative-transfer calculations using Cloudy , comparing the results to the SINFONI Br γ profile of NGC 3783. We find that Br γ arises from ionized gas in the surface of the rotating thin disk, with electron temperatures of approximately T e ≈ 10 4 K and number densities of n e ≈ 10 8 –10 11 cm −3 . However, the intrinsic line profile produced by the RHD kinematics is narrower than observed and displays a substructure. An approximate treatment of the electron scattering suggests that scattering in the surrounding diffuse ionized gas significantly broadens and smooths the intrinsic Br γ profile, making it consistent with the observed profile. This scattering medium has an electron temperature of 10 4 –10 5 K and a number density of n ≲ 10 8 cm −3 . Although a best-fit viewing angle of ≈15° is suggested, the scattered line is notably less sensitive to inclination than the intrinsic line. The observed BLR profiles may be understood as the intrinsic emission viewed through an electron-scattering haze, such that some spectral detail is plausibly redistributed rather than seen directly.