Reverberation Mapping Data of NGC 5548 Imply a Multicomponent Broad-Line Region

TL;DR

This study analyzes reverberation mapping data of NGC 5548 and finds that simple single-component models cannot explain the observed emission line profiles, suggesting a multicomponent broad-line region structure.

Contribution

The paper provides evidence that the broad-line region in NGC 5548 is likely composed of multiple components, challenging simpler existing models.

Findings

Single-peak broad-line profiles with double-peak delay profiles observed

Simple cloud or thin disk models cannot fully explain the data

Multicomponent models may better represent the broad-line region

Abstract

Line broadening and variability are observational hallmarks of active galactic nuclei which allow us to measure supermassive black hole masses as well as constrain the geometry and kinematics of the emitting gas, with the most precise measurements requiring a degree of modeling. Two popular models of the broad-line region describe the emitting gas as either a distribution of puffed up clouds or a thin disk with strong velocity gradients. As we show in this work, key features in the reverberation mapping dataset obtained by the AGN STORM team in NGC 5548 cannot be accounted for by either simple model. In several emission lines the observed broad-line profile has a single peak yet the delay profile has a distinct double peak, strongly motivating a BLR with emission from multiple components. We demonstrate a few possibilities that may alleviate the tension and better represent the true nature of the broad-line emitting gas in NGC 5548 and beyond.