Dynamics of embedded bars and the connection with AGN^*

I. ISAAC/VLT stellar kinematics

¹ Centre de Recherche Astronomique de Lyon, 9 Av. Charles André, 69561 Saint-Genis Laval Cedex, France
² Observatoire de Genève, 1290 Sauverny, Switzerland
³ DEMIRM, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
⁴ Gymnase de Nyon, 1260 Nyon, Switzerland
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 1-87, Nankang, Taipei, Taiwan, R.O.C.
⁶ Observatoire de Marseille-Provence, Laboratoire d'Astrophysique de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 4, France

Corresponding author: E. Emsellem, emsellem@obs.univ-lyon1.fr

Received: 27 October 2000
Accepted: 19 December 2000

Abstract

We present new stellar kinematical profiles of four galaxy hosts of active galactic nuclei, using the ¹²CO bandhead around 2.3 μm with the ISAAC/VLT spectrograph. We find that the nuclear bars or discs, embedded in large-scale primary bars, have all a decoupled kinematics, in the sense that the maximum of the rotational velocity occurs in the nuclear region. In three cases (NGC 1097, NGC 1808 and NGC 5728), the velocity dispersion displays a significant drop at the nucleus, a rarely observed phenomenon. We also detect kinematical asymmetries ( mode) along the nuclear bar major-axis of NGC 1808 and NGC 5728, dynamical counterparts of corresponding asymmetries in the surface brightness. We have derived simple dynamical models in an attempt to fit the kinematics of each galaxy and reconstruct the full velocity field. For all four targets, the fits are good, and confirm the presence of the decoupled nuclear components. These models cannot however reproduce the observed central drop in the dispersion. We suggest that this drop is due to a transient cold nuclear disc, fuelled by gas inflow along the bar, that has recently formed new stars.