Multi-line reverberation mapping uncovers hidden dynamics in active galactic nuclei

A research team from the Yunnan Observatories of the Chinese Academy of Sciences has recently published a study in The Astrophysical Journal that examines the inner and outer broad-line regions (BLRs) of active galactic nuclei (AGNs).

The BLR is a crucial component of AGNs, characterized by gas clouds moving rapidly under the influence of a supermassive black hole (SMBH). This motion creates broad emission lines through Doppler broadening. Understanding the BLR’s geometry and kinematics is essential for insights into black hole environments and mass measurements.

Traditionally, studies have focused on a single emission line for reverberation mapping (RM), limiting comprehension of the BLR’s gas distribution and dynamics.

To address this limitation, the researchers introduced a novel method using multiple emission lines for RM observations. They applied the method to two variable AGNs: KUG 1141+371 and UGC 3374.

Their findings revealed significant radial ionization stratification and distinct kinematic behaviors in the inner and outer BLRs. For example, in KUG 1141+371, the inner BLR exhibited outflowing motion, while the outer BLR was virialized.

In contrast, UGC 3374 showed inflowing motion in the outer BLR and a virialized inner BLR. The researchers also identified a transition from “breathing” (expansion and contraction) to “anti-breathing” (inverse motion) behavior between the inner and outer BLR, indicating distinct physical properties.

To interpret these findings, the team explored various possible scenarios for BLR formation, including the roles of radiation pressure, magnetically driven winds, and the tidal disruption of dusty gas clouds.

The observed geometric and kinematic stratification helps to explain the rapid changes in velocity-resolved RM signals noted in recent studies, as well as the variability in black hole mass estimates obtained from different emission lines or luminosities.

This research underscores the importance of long-term, multiline RM observations to unravel the complex geometry and evolution of the BLR in AGNs.