Recently, postdoc LI Shasha from Yunnan Observatories of the Chinese Academy of Sciences, and their collaborators, developed a new technique to find the complex geometry and kinematics of broad-line region (BLR) in active galactic nuclei (AGN). The related work, titled "Velocity-Resolved Ionization Mapping of Broad Line Region. I. Insights into Diverse Geometry and Kinematics," was published in The Astrophysical Journal. 

The BLR is widely believed to consist of ionized gas moving at high velocities under the gravitational influence of the central supermassive black hole (SMBH). This gas emits broad spectral lines, making the study of its spatial distribution and dynamical behavior crucial for understanding the internal structure of AGNs and accurately measuring the mass of the black hole.

Reverberation mapping (RM) is commonly used to study the geometry and kinematics of the BLR by measuring time lags between AGN continuum and broad emission line variations. Traditionally, the BLR is assumed to have symmetric geometry, with Keplerian motion producing symmetric velocity-resolved lags. Asymmetric RM signatures are attributed to inflow or outflow motions. However, if the BLR has an elliptical structure with the SMBH at one focus, lags at the apoapsis would naturally be longer than at the periapsis, even with Keplerian motion, leading to degeneracies in interpreting the BLR's true geometry and kinematics. 

In this study, researchers proposed that one can study geometry and kinematics of BLR by mapping the distribution of its ionization properties, and they have named this method "ionization mapping" (IM). Since gas at different distances from the center is subjected to varying levels of incident photons, by analyzing the ionization state of the gas at these locations, researchers can infer the distance of the gas from the center, thereby mapping out the detailed geometrical and kinematic properties of the BLR.

The ionization properties of the gas are typically traced by the emission line flux ratios. The researchers discovered a significant anti-correlation between the ratio of Balmer lines (also known as the Balmer decrement; BD) and the continuum intensity. This finding suggests that gas farther from the center exhibits a larger BD value. As the distance from the central ionizing source increases, the continuum flux decreases exponentially, leading to a rapid reduction in the incident ionizing flux on the gas. This method, which does not rely on the light travel time, can be combined with RM to effectively disentangle the degeneracy between the BLR’s geometry and kinematics.

By comparing the RM and IM measurements, the researchers found a strong correlation between the lags and BDs at different velocities of BELs, indicating that regions with longer lags are probably farther from the center. This result suggests that the BLR of AGNs is more complex than previously expected, necessitating the development of more detailed physical models.

Contact:
FENG Haicheng
Yunnan Observatories, CAS
E-mail: hcfeng@ynao.ac.cn