Broad line regions behind haze: Intrinsic shape of Br$γ$ line and its origin in a type-1 Seyfert galaxy

TL;DR

This study tackles how broad-line region spectra in AGNs arise by coupling three-dimensional radiation-hydrodynamic simulations of gas near a supermassive black hole with detailed radiative-transfer calculations for Br$\gamma$ in NGC 3783. The intrinsic emission from a surface layer of a rotating thin disk is found to be narrowed and structured, but Thomson scattering in a diffuse, ionized haze with $T_e\sim10^4-10^5$ K and $n_e\lesssim10^8$ cm$^{-3}$ broadens and smooths the profile to resemble the observed Br$\gamma$ line, with a best-fit viewing angle around $\theta_v\approx15^{\circ}$. The results indicate that observed BLR line shapes may reflect propagation effects in a surrounding scattering medium rather than solely intrinsic kinematics, which has important implications for SMBH mass estimates and BLR geometry in type-1 AGNs. The work also emphasizes the need for fully self-consistent 3D radiative transfer, including scattering, to robustly connect BLR emission to the underlying gas dynamics.

Abstract

The broad-line region (BLR) of active galactic nuclei (AGN) is an essential component, yet its small size keeps its origin, structure, and kinematics uncertain. Infrared interferometry with VLTI/GRAVITY is now resolving 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-principles 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 $T_e \approx 10^4$ K and number densities of $n_e \approx 10^8-10^{11}$ cm$^{-3}$. However, the intrinsic line profile produced by the RHD kinematics is narrower than observed and displays substructure. An approximate treatment of the electron scattering suggests that scattering in 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 $n \lesssim 10^8$ cm$^{-3}$. Although a best-fit viewing angle of $\approx 15$ deg 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.

Figures (10)

• Figure 1: Density slice and temperature slices with velocity vectors in the $x-z$ plane at $y = 0$ pc.
• Figure 2: (left) Density--temperature plot (colored by radius). (right) Three-dimensional distribution of the "BLR candidate" grid cells, which correspond to the cells in the red box of the density-temperature plot.
• Figure 3: The non-spherical SED of the central source for $i = 30,42, 54, 66, 78$, and 90$^\circ$.
• Figure 4: The dispersion of the exponential tail by the electron scattering as a function of the optical depth $\tau_e$ (Equations \ref{['eq: exp_tail']} and \ref{['eq: sigma']}) for three electron temperatures $T_e$Laor2006.
• Figure 5: Electron--scattering optical depth map, $T>8000$ K. The white contour represents the photosphere ($\tau_e = 1$).