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Astron. Astrophys. 364, 443-449 (2000)

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4. Implications for the AGN models

As shown in Fig. 1 the luminosity distribution of the X-ray selected AGN has a FWHM of about 5 mag., corresponding to a factor 100. On the other hand Fig. 3 shows a FWHM of the X-ray/visual flux ratio of only a factor of [FORMULA] 10. This seems to indicate that the basic physical conditions of the X-ray and visual continuum emitting volumes of the AGN are similar and are not strongly dependent on the luminosity. Moreover, the fact that the X-ray selected Seyfert 1s and 2s have similar mean [FORMULA] values supports the assumption of a similar physical structure of all Seyfert nuclei and QSOs.

Assuming that the H[FORMULA] FWZI corresponds to about twice the maximum orbital velocity of the BLR plasma, we obtain for our sample velocities of 1000 [FORMULA] kms-1. If these values correspond to circular orbits in the potential of a black hole, the observed velocity range can be converted into a radial distance range for the BLRs of [FORMULA] (where [FORMULA] is the BH's Schwarzschild radius). The observed particularly high BLR line widths for some low-luminosity Seyfert 1s and the absence of such high velocities for the more luminous QSOs (Fig. 8) could in this case be explained by the fact that the intense radiation field of the luminous QSOs prevents the existence of relatively cool Balmer line emitting BLR gas at low [FORMULA] values, while at low luminosities the BLR gas can exist in a large range of distances. Less clear is the interpretation of the NLS1 in this scheme. (For a recent compilation of proposed explanations see Komossa & Janek 2000). While the assumption of a low central mass being the origin of the absence of broad line components (Boller et al. 1996; Laor 2000) seems plausible, other explanations cannot be ruled out on the basis of the present data.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001