Introduction to active galaxies
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# Broad-line region

In the model, the clouds of the broad-line region surround the central engine within the opening in the middle of the dust torus. The radius of the BLR is of the order of 1014 m, placing it well inside the torus. At this distance from the black hole orbital speeds are several thousand kilometres per second, which is consistent with the typical speed of 5000 km s−1 that is measured from Doppler broadening. The clouds are fully exposed to the intense radiation from the engine (remember that any dust will have vaporised in this region) and will be heated to a high temperature. It is difficult to measure the temperature of BLR clouds, but it appears to be of the order of 104 K.

It has been estimated that the BLR of a typical AGN will have about 1010 clouds covering about 10% of the sky as seen from the central engine. The total mass of gas is less than 10M ⊙, so it is utterly negligible compared with the black hole itself.

Broad lines are not seen in every AGN. The general belief among astronomers is that every AGN has a broad-line region, but in some cases our view of the BLR clouds is obscured by the dust torus, so broad lines do not appear in the spectrum.

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