An evolving view of the BLR

The consensus view of AGN structure has remained fairly stable, if somewhat vague and lacking in details and self-consistency, for over 15 years. During that period, work has steadily progressed, and there have been some long-standing issues on which consensus is beginning to emerge. The following constitutes the author's personal view of some of the changes that are beginning to take hold.

The notion that the BLR is comprised of a large number of discrete gas clouds can be traced back to the early days of AGN research, when the best conceptual model of the BLR structure was a collection of optically thick clouds or filaments - in other words, something that might look like a large version of the Crab Nebula. The number of clouds or filaments required to account for the observed characteristics of the BLR turns out to be very large.

It was recognized long ago that the smoothness of broad-line profiles implies that there must be a lower limit (> 105 clouds) to the number of line-emitting clouds if the intrinsic line profile of each cloud is thermal (Capriotti et al. 1981; Atwood et al. 1982). This lower

5.9.1 The BLR gas: discrete clouds or a continuous flow?

Emissions Like That Horse

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