Issue |
A&A
Volume 441, Number 3, October III 2005
|
|
---|---|---|
Page(s) | 1011 - 1030 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20052900 | |
Published online | 23 September 2005 |
Molecular gas in NUclei of GAlaxies (NUGA)
IV. Gravitational torques and AGN feeding
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
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 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
© ESO, 2005
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