Observational constraints on the internal velocity field of quasar emission-line clouds

This paper addresses the question, what does the spectrum of a typical quasar reveal about the velocity structure within its broad emission line region clouds? Turbulent (i.e., nonthermal) broadening of spectral lines can be due to macroturbulence or microturbulence. Microturbulence affects line formation and the emitted spectrum and may be required to account for the observed smoothness of the line profiles. The velocity field is crucial since it addresses the fundamental nature of the individual clouds and the global structure of the active galactic nuclei (AGNs) environment. For example, stellar winds or magnetically confined blobs might be highly microturbulent, requiring only a few internally broadened clouds to account for the observed smooth line profiles in AGNs. On the other hand, clouds in pressure confinement would have only thermal line widths, requiring many clouds moving in a large-scale velocity field to achieve the same effect. There are almost no previous studies of the effects of microturbulence, even though the observation that AGN lines are very smooth seems to require additional line broadening mechanisms. We present a broad range of photoionization calculations in which the microturbulence is varied between 0 km s^-1 (thermal broadening only) and 10⁴ km s^-1, an upper limit set by the observed line width. In general, the line spectrum grows stronger relative to the continuum as turbulence increases. This is because lines more easily escape due to diminished line optical depth and permitted lines are selectively strengthened by continuum pumping. Comparisons with observations reveal two cases. The predicted relative intensities of the majority of the strong lines in typical objects do not depend strongly on the microturbulent field. A turbulence of ∼10³ km s^-1 does not violate observations, but is not required either. However, in the sharp-lined quasars, some lines require a turbulence of the same order as the observed line width to reproduce the spectrum.