Molecular gas in NUclei of GAlaxies (NUGA) -: IV.: Gravitational torques and AGN feeding

We discuss the efficiency of stellar gravity torques as a mechanism to account for the feeding of the central engines of four low luminosity Active Galactic Nuclei (AGN): NGC 4321 (HII nucleus/LINER), NGC 4826 (HII nucleus/LINER), NGC 4579 (LINER 1.9/Seyfert 1.9) and NGC 6951 (Seyfert 2). These galaxies have been observed as part of the Nuclei of Galaxies-(NUGA) CO project, aimed at the study of AGN fueling mechanisms. Our calculations allow us to derive the characteristic time- scales for gas flows and discuss whether torques from the stellar potentials are efficient enough to drain the gas angular momentum in the inner 1 kpc of these galaxies. The stellar potentials are derived using high- resolution near infrared (NIR) images and the averaged e. ffective torques on the gas are estimated using the high- resolution (similar to 0.5 ''-2 '') CO maps of the galaxies. Results indicate paradoxically that feeding should be thwarted close to the AGNs: in the four cases analyzed, gravity torques are mostly positive inside r similar to 200 pc, resulting in no inflow on these scales. As a possible solution for the paradox, we speculate that the agent responsible for driving inflow to still smaller radii is transient and thus presently absent in the stellar potential. Alternatively, the gravity torque barrier associated with the Inner Lindblad Resonance of the bars in these galaxies could be overcome by other mechanisms that become competitive in due time against gravity torques. In particular, we estimate on a case-by-case basis the e. fficiency of viscosity versus gravity torques to drive AGN fueling. We find that viscosity can counteract moderate-to-low gravity torques on the gas if it acts on a nuclear ring of high gas surface density contrast and similar to a few 100 pc size. We propose an evolutionary scenario in which gravity torques and viscosity act in concert to produce recurrent episodes of activity during the typical lifetime of any galaxy. In this scenario the recurrence of activity in galaxies is indirectly related to that of the bar instabilities although the active phases are not necessarily coincident with the maximum strength of a single bar episode. The general implications of these results for the current understanding of fueling of low-luminosity AGN are discussed.