Free access article

A&A 372, 50-58 (2001)
DOI: 10.1051/0004-6361:20010475

Size-mass-luminosity relations in AGN and the role of the accretion disc

S. Collin¹ and J.-M. Huré<sup>1, 2

1</sup>  DAEC/UMR 8631 du CNRS, Observatoire de Paris, Section de Meudon, 92195 Meudon, France
²  Université Paris 7 Denis Diderot, Place Jussieu, 75251 Paris Cedex 05, France

(Received 2 January 2001 / Accepted 21 March 2001)

Abstract
We address the question of the relations between the black hole's mass, the accretion rate, the bolometric luminosity, the optical luminosity and the size of the Broad Line Region (BLR) in Active Galactic Nuclei, using recent observational data obtained from monitoring campaigns. We first show that a standard accretion disc cannot account for the observed optical luminosity, unless it radiates at super-Eddington rates. This implies the existence of another, dominant emission mechanism in the optical range, which could be due to the reprocessing of X-rays by a system of dense clouds, or a non standard disc (non stationary, ADAF and/or strong outflows). Narrow Line Seyfert 1 galaxies (NLS1s) are most extreme in this context: they have larger bolometric to Eddington luminosity ratios than Broad Line Seyfert 1 (BLS1s), and most likely a larger "non disc" component in the optical range. Second, from realistic simulations of self-gravitating -discs, we have systematically localized the gravitationally unstable disc and shown that, given uncertainties on both the model and observations, it coincides quite well with the size of the BLR. We therefore suggest that the gravitationally unstable disc is the source which releases BLR clouds in the medium. However, the influence of the ionization parameter is also required to explain the correlation found between the size of the BLR and the luminosity. In this picture, the size of the BLR in NLS1s (relative to the black hole size) is larger (and the emission line width smaller) than in BLS1s simply because their Eddington ratio is larger.

Key words: accretion, accretion disks -- instabilities -- galaxies: active -- galaxies: nuclei

Offprint request: S. Collin, suzy.collin@obspm.fr

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