Magnetohydrodynamic stability of broad line region clouds

TL;DR

This study investigates the magnetohydrodynamic stability of broad line region clouds in active galactic nuclei, showing that magnetic fields can significantly influence cloud stability and fragmentation.

Contribution

It introduces MHD simulations of AGN clouds with different magnetic configurations, revealing the stabilizing effects of helical fields and the instability caused by azimuthal fields.

Findings

Magnetic fields of a few Gauss are relevant for cloud stability.

Azimuthal magnetic fields lead to cloud fragmentation and dispersal.

Helical magnetic fields can stabilize cloud cores.

Abstract

Hydrodynamic stability has been a longstanding issue for the cloud model of the broad line region in active galactic nuclei. We argue that the clouds may be gravitationally bound to the supermassive black hole. If true, stabilisation by thermal pressure alone becomes even more difficult. We further argue that if magnetic fields should be present in such clouds at a level that could affect the stability properties, they need to be strong enough to compete with the radiation pressure on the cloud. This would imply magnetic field values of a few Gauss for a sample of Active Galactic Nuclei we draw from the literature. We then investigate the effect of several magnetic configurations on cloud stability in axi-symmetric magnetohydrodynamic simulations. For a purely azimuthal magnetic field which provides the dominant pressure support, the cloud first gets compressed by the opposing radiative and gravitational forces. The pressure inside the cloud then increases, and it expands vertically. Kelvin-Helmholtz and column density instability lead to a filamentary fragmentation of the cloud. This radiative dispersion continues until the cloud is shredded down to the resolution level. For a helical magnetic field configuration, a much more stable cloud core survives with a stationary density histogram which takes the form of a power law. Our simulated clouds develop sub-Alfvenic internal motions on the level of a few hundred km/s.