Space Telescope and Optical Reverberation Mapping Project. XII. Broad-Line Region Modeling of NGC 5548

TL;DR

This study models the broad-line region of NGC 5548 using multi-wavelength reverberation mapping data, revealing its geometry, kinematics, and black hole mass, and comparing different epochs to understand BLR evolution.

Contribution

It provides detailed geometric and dynamical models of the BLR for NGC 5548, including multi-line analysis and comparison across different luminosity states, advancing understanding of BLR structure and black hole mass measurement.

Findings

The Hβ BLR is disk-like with near-circular and outflowing gas.

C IV and Lyα BLRs are shell-like and smaller in radius.

Black hole mass is consistently estimated around 10^7.64 solar masses.

Abstract

We present geometric and dynamical modeling of the broad line region for the multi-wavelength reverberation mapping campaign focused on NGC 5548 in 2014. The dataset includes photometric and spectroscopic monitoring in the optical and ultraviolet, covering the H$\beta$, C IV, and Ly$\alpha$ broad emission lines. We find an extended disk-like H$\beta$ BLR with a mixture of near-circular and outflowing gas trajectories, while the C IV and Ly$\alpha$ BLRs are much less extended and resemble shell-like structures. There is clear radial structure in the BLR, with C IV and Ly$\alpha$ emission arising at smaller radii than the H$\beta$ emission. Using the three lines, we make three independent black hole mass measurements, all of which are consistent. Combining these results gives a joint inference of $\log_{10}(M_{\rm BH}/M_\odot) = 7.64^{+0.21}_{-0.18}$. We examine the effect of using the $V$ band instead of the UV continuum light curve on the results and find a size difference that is consistent with the measured UV-optical time lag, but the other structural and kinematic parameters remain unchanged, suggesting that the $V$ band is a suitable proxy for the ionizing continuum when exploring the BLR structure and kinematics. Finally, we compare the H$\beta$ results to similar models of data obtained in 2008 when the AGN was at a lower luminosity state. We find that the size of the emitting region increased during this time period, but the geometry and black hole mass remain unchanged, which confirms that the BLR kinematics suitably gauge the gravitational field of the central black hole.