Starbursts and torus evolution in AGN

¹ CDS, Observatoire astronomique de Strasbourg, UMR 7550, 11 rue de l'université, 67000 Strasbourg, France e-mail: bvollmer@astro.u-strasbg.fr
² Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Max Planck Insitut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

Received: 23 June 2008
Accepted: 15 September 2008

Abstract

Recent VLT SINFONI observations of the close environments (~30 pc) of nearby AGNs have shown that thick gas tori and starbursts with ages between 10 and 150 Myr are frequently found. By applying these observations to a previously established analytical model of clumpy accretion disks, we suggest an evolutionary sequence for starburst and AGN phases. Whereas the observed properties of the gas tell us about the current state of the torus, the starburst characteristics provide information on the history of the torus. In the suggested evolution, a torus passes through 3 different phases predetermined by an external mass accretion rate. Started by an initial, short, and massive gas infall, a turbulent and stellar wind-driven disk is formed in which the starburst proceeds. Once the supernovae explode the intercloud medium is removed, leaving a massive, geometrically thick, collisional disk with a decreasing, but still high-mass accretion rate. When the mass accretion rate has significantly decreased, the collisional torus becomes thin and transparent as the circumnuclear disk in the Galactic center of the Milky Way. Variations on this scenario are possible either when there is a second short and massive gas infall, in which case the torus may switch back into the starburst mode, or when there is no initial short massive gas infall. All observed tori up to now have been collisional and thick. The observations show that this phase can last more than 100 Myr. During this phase the decrease in the mass accretion rate within the torus is slow (a factor of 4 within 150 Myr). The collisional tori also form stars, but with an efficiency of about 10% when compared to a turbulent disk.