Accretion Flows in Active Galaxies

Accretion of matter onto a massive black hole is believed to be the power source of Acti've Galactic Nuclei (AGN). Th.e idea that its ultimate manifestation is the release of g.ravitational energy in an accretion disk around the black hole has gained wide accepta11ce in the scientifi.c community, although many aspects remain open to question. The main reason for t_his resides in the very limited spatial extent of the central engine prnd11cing the power output of AGN, ;S lpc, so that direct resolution of the accretion disk is far beyond our capabilities even in the nearest AGN. Since there is probably not enough supply of matter available in situ in the galactic nucleus, the accretion. flow should be triggered in the bulge or in the disk of the galaxy. There is no general consensus as far as the triggering mechanism is concerned. We have thus focussed our investigation. on the origin of the accretion flow, and on its ultimate stage - the accretion disk. lHaterial at a non-trivia.I rate of rv 0.1-10 M0 yr-1 (depending on the AGN luminosity) should be able to reach the central black hole. It is unclear how the galactic gas can loose its angular momentum, drift toward the cent er of the galaxy, ultimately reach the central parsec of the active nucleus to provide its fuel. In order to study the relevance of interaction with a close companion galaxy in. driving gravitation.al instabilities able to channel the gas flow toward the active nucleus, we have analyzed at fi.rst the frequency of companion galaxies among Seyfert galaxies (Chapter 2). The path of the accretion. flow is complicated by the occurrence of dissipative processes which produce not only radial drift of gas, but also enhance the star formation rate, producing a Starburst in the most extreme cases. There is evidence suggesting a link between interaction, bursts of star formation, and fueling of AGN, but it is not clear whether a Starburst is a necessary preliminary for the birth of an AGN. Intermediate objects, which show features of a Star burst as well as of a Seyfert galaxy, and at the same time belong to an interacting system, are ideal laboratories in order to an.alyze the effects of interaction on the star formation rate and on the gas kinematics, and any connection with the nuclear activity. In Chapter 3, we study in detail the spectroscopic properties of one of these systems, NGC 7592. The ultimate stage of the accretion flow is probably to build up an accretion disk. If the accretion rate is above a critical value, the accretion flow may become geometrically thick, and the radiation emitted would be probably anisotropic. We have pursued a test based on the analysis of the profiles of the broad low ionization profile to search for an unmistakable signature of the accretion disk. This test has been concentrated on some selected best candidates as well as on a statistical analysis of the emission line profile for a. large sample of AGN (Chapter 4). In Chapter 5 we have investigated the effect that anisotropic emission from a thick accretion disk may have on the luminosity function of the AGN, and on the estimate of the central masses. In the first Chapter we review some basic aspects of AGN, and we discuss the present status of the art on the problems pointed above. In the concluding remarks (Chapter 6) the original contributions obtained in this work are summarized, and plans for further investigations are discussed.