Dynamics of embedded bars and the connection with AGN - I. ISAAC/VLT stellar kinematics

We present new stellar kinematical profiles of four galaxy hosts of active galactic nuclei, using the (CO)-C-12 bandhead around 2.3 mum with the ISAAC /VLT spectrograph. We find that the nuclear bars or discs, embedded in larg e-scale primary bars, have all a decoupled kinematics, in the sense that th e 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 de tect kinematical asymmetries (m = 1 mode) along the nuclear bar major-axis of NGC 1808 and NGC 5728, dynamical counterparts of corresponding asymmetri es in the surface brightness. We have derived simple dynamical models in an attempt to fit the kinematics of each galaxy and reconstruct the full velo city field. For all four targets, the fits are good, and confirm the presen ce of the decoupled nuclear components. These models cannot however reprodu ce the observed central drop in the dispersion. We suggest that this drop i s due to a transient cold nuclear disc? fuelled by gas inflow along the bar , that has recently formed new stars.