Magnetic confinement, magnetohydrodynamic waves and smooth line profiles in active galactic nuclei

In this paper, we show that if the broad-line region clouds are in approxim ate energy equipartition between the magnetic field and gravity, as hypothe sized by Rees, there will be a significant effect on the shape and smoothne ss of broad emission-line profiles in active galactic nuclei. Linewidths of contributing clouds or flow elements are much wider than their thermal wid ths, because of the presence of non-dissipative magnetohydrodynamic waves a nd their collective contribution produce emission-line profiles broader and smoother than would be expected if a magnetic field were not present. As a n illustration, a simple model of isotropically emitting clouds, normally d istributed in velocity, is used to show that smoothness can be achieved for less than similar to 8 x 10(4) clouds and may even be as low as a few hund red. We conclude that magnetic confinement has far-reaching consequences fo r observing and modelling active galactic nuclei.