Prof. LIU Hongtao, PhD. FENG Haicheng and their cooperators, from Yunnan observatories of Chinese Academy of Sciences, investigated the virial factor in estimating the virial masses of supermassive black holes in active galactic nuclei (AGNs) through the redward shifted broad emission lines Hβ and Fe II. Recently, their research was published online in The Astrophysical Journal.

It is widely believed that the virial factor is only determined by the gravitationally bound configurations of broad-line regions (BLRs) in AGNs. It may actually be the opposite. The accurate measurement of black hole mass is important to investigate the formation and evolution of galaxy and black hole.

Based on the method proposed by LIU etl. in 2017 and the line width and velocity shift of the redward shifted Hβ and Fe II for 1973 SDSS DR5 quasars, the researchers measured the virial factor in determining the supermassive black hole masses, usually estimated by the reverberation mapping (RM) method or the relevant secondary methods, which use the average virial factor derived from the other methods for reverberation mapped nearby AGNs. A larger systematic uncertainty of mass is caused by using of the average to replace the actual value.

Positive correlation is found between the virial factor and dimensionless accretion rate of black hole, and this finding is consistent with the prediction of considering radiation pressure force. There is a three-dimensional positive correlation of the virial factor with the dimensionless accretion rate and line ratio Fe II/Hβ, indicating that the virial factor is likely dominated by the dimensionless accretion rate and metallicity.

A negative correlation is found between the redward shift of Hβ and the scaled size of broad-line region radius in units of the gravitational radius of black hole. This negative correlation will be expected naturally if the redward shift of Hβ is mainly from the gravity of black hole. Radiation pressure force from accretion disk radiation is a significant contributor to the virial factor.

These results show that the radiation pressure force on the gravitationally bound BLRs of AGNs is prevalent and non-negligible. These new findings enable people to better understand the macroscopic effects of microphysical processes, and the physical structures and mechanisms in AGNs.

Contact:

LIU Hongtao, Yunnan Observatories, CAS

E-mail: htliu@ynao.ac.cn