Dissipative Heating and Quasar Emission Lines

Abstract

Recent observations reveal that the profiles of emission lines of active galactic nuclei are too smooth to be produced by discrete thermal (T � 10 4 K) clouds. The lines may also be too bright to be powered by the continuum unless a large covering factor or additional heating mechanisms are present. We have been investigating one possible explanation of these observations, namely, that the clouds are turbulent. This paper focuses on observational effects caused by dissipation of turbulent energy into cloud heating. We find that internal heating can explain these observations. Clouds energized by both the ionizing continuum and dissipative heating are more efficient line emitters than those powered by the continuum alone. The turbulent velocity field broadens the emission contributions of individual line-emitting clouds so that they overlap, smoothing the line profile. We have broad success in reproducing the observed emission-line spectrum with a turbulent velocity of � 200 km s � 1 , a cloud density of 10 10 cm � 3 , and a column density of 10 22 cm � 2 . Dissipative turbulence selectively increases intensities of low-ionization lines, making it possible to obtain the ‘‘ standard ’’ broad-line region line spectrum with a column density � 10 times smaller than usually assumed. The presence of dissipative heating could explain two long-standing puzzles in quasar emission-line spectra, namely, the smooth line profiles and the energetics of the spectrum. Subject headings: MHD — quasars: emission lines — quasars: general