Broad-line region geometry from multiple emission lines in a single-epoch spectrum

¹ Max Planck Institute for Extraterrestrial Physics (MPE), Giessenbachstr. 1, 85748 Garching, Germany
e-mail: shangguan@mpe.mpg.de
² Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada
³ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
⁴ Department of Astrophysical & Planetary Sciences, JILA, University of Colorado, Duane Physics Bldg., 2000 Colorado Ave, Boulder, CO 80309, USA
⁵ Department of Physics, Technical University of Munich, James- Franck-Straße 1, 85748 Garching, Germany
⁶ Departments of Physics & Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
⁷ Department of Physics and Astronomy, University of Southampton, Southampton, UK
⁸ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel

Received: 3 October 2023
Accepted: 22 January 2024

Abstract

The broad-line region (BLR) of active galactic nuclei (AGNs) traces gas close to the central supermassive black hole (BH). Recent reverberation mapping (RM) and interferometric spectro-astrometry data have enabled detailed investigations of the BLR structure and dynamics as well as estimates of the BH mass. These exciting developments have motivated comparative investigations of BLR structures using different broad emission lines. In this work, we have developed a method to simultaneously model multiple broad lines of the BLR from a single-epoch spectrum. We applied this method to the five strongest broad emission lines (Hα, Hβ, Hγ, Paβ, and He Iλ5876) in the UV-to-near-IR spectrum of NGC 3783, a nearby Type I AGN that has been well studied by RM and interferometric observations. Fixing the BH mass to the published value, we fit these line profiles simultaneously to constrain the BLR structure. We find that the differences between line profiles can be explained almost entirely as being due to different radial distributions of the line emission. We find that using multiple lines in this way also enables one to measure some important physical parameters, such as the inclination angle and virial factor of the BLR. The ratios of the derived BLR time lags are consistent with the expectation of theoretical model calculations and RM measurements.