Nuclear Components of Active Galaxies

In the following the most relevant nuclear components of active galaxies are described. Figure 22.1 gives a schematic illustration. Within the centre, the hypothetical black hole is located, surrounded by an accretion disk. At larger distances from the central black hole are located, the region where the broad emission lines (Broad Line Region, BLR) originate and the region of the narrow emission lines (Narrow Line Region, NLR). Another component detected in some active galaxies is the molecular torus, which is probably located within the NLR. The molecular torus leads to strong absorption of the emission from the nuclei of active galaxies. It is

Fig. 22.1 Schematic illustration of the principal components in the nuclei of active galaxies. The black hole is surrounded by an accretion disk. The radiation from the accretion disk, which is considered as the primary energy source for the excitation of the optical emission lines in active galaxies, is marked by the arrows. The accretion disk is surrounded by the Broad Line Region (BLR). A system of relatively cool (about 104 K) clouds (marked by the dashed circles), thought to emit the broad emission lines in active galaxies, is probably embedded in a hot (about 108 K) ionized gas. Both components are in pressure equilibrium. Further out is seen the region of the primary origin of the narrow emission lines (NLR), marked by the dashed ellipses

Fig. 22.1 Schematic illustration of the principal components in the nuclei of active galaxies. The black hole is surrounded by an accretion disk. The radiation from the accretion disk, which is considered as the primary energy source for the excitation of the optical emission lines in active galaxies, is marked by the arrows. The accretion disk is surrounded by the Broad Line Region (BLR). A system of relatively cool (about 104 K) clouds (marked by the dashed circles), thought to emit the broad emission lines in active galaxies, is probably embedded in a hot (about 108 K) ionized gas. Both components are in pressure equilibrium. Further out is seen the region of the primary origin of the narrow emission lines (NLR), marked by the dashed ellipses assumed, that the main difference between Seyfert 1 and Seyfert 2 galaxies is due to orientation effects. The Full-Width at Half Maximum (FWHM) of the Balmer lines in Seyfert 1 galaxies reaches values up to about 100000 km s-1, in contrast to Seyfert 2 galaxies where the typical FWHM values are of the oder of only a few hundred km s^1. Absorption along the line of sight, caused by the molecular torus can explain the absence of broad lines in Seyfert 2 galaxies. Part of the emission of the central source is scattered in the observers direction. This leads to polarization and the polarized part of the emission of Seyfert 2 galaxies shows the typical broad emission lines as observed in Seyfert 1 galaxies [1].

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