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broad emission line region

(broad emission line cloud, BEL region, BEL cloud)
(region within an AGN with broad emission lines)

A broad emission line region (or broad emission line cloud or BEL region/cloud) is a portion of an active galactic nucleus showing broad emission lines, presumed to be Doppler broadening, indicating glowing material with a high velocity dispersion. The regions are presumed to be near the supermassive black hole and its accretion disk, the apparent velocity presumed to be due to angular momentum of material moving toward th black hole, i.e., material being drawn, but not (yet) in the flat disk. Typical is a larger, surrounding region with narrow emission lines (narrow emission line region or NEL region).

Reverberation mapping is a technique for estimating the SMBH's mass through characteristics of such a BEL region.