EDP Sciences
Free access
Issue
A&A
Volume 441, Number 3, October III 2005
Page(s) 1011 - 1030
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20052900


A&A 441, 1011-1030 (2005)
DOI: 10.1051/0004-6361:20052900

Molecular gas in NUclei of GAlaxies (NUGA)

IV. Gravitational torques and AGN feeding
S. García-Burillo1, F. Combes2, E. Schinnerer3, F. Boone4 and L. K. Hunt5

1  Observatorio Astronómico Nacional (OAN) - Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
    e-mail: s.gburillo@oan.es
2  Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France
    e-mail: francoise.combes@obspm.fr
3  Max-Planck-Institut für Astronomie, Königstuhl, 17, 69117 Heidelberg, Germany
    e-mail: schinner@mpia-hd.mpg.de
4  Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germany
    e-mail: fboone@mpifr-bonn.mpg.de
5  Istituto di Radioastronomia/CNR, Sez. Firenze, Largo Enrico Fermi, 5, 50125 Firenze, Italy
    e-mail: hunt@arcetri.astro.it

(Received 18 February 2005 / Accepted 24 June 2005 )

Abstract
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 effective torques on the gas are estimated using the high-resolution (~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 $r\sim 200$ 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 efficiency 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 ~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.


Key words: galaxies: individual: NGC 4321, NGC 4579, NGC 4826, NGC 6951 -- galaxies: ISM -- galaxies: kinematics and dynamics -- galaxies: nuclei -- galaxies: Seyfert -- radio lines: galaxies

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