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

TL;DR

This study analyzes stellar kinematics in four active galaxy nuclei, revealing decoupled nuclear components and transient cold nuclear discs likely formed by bar-driven gas inflow, with implications for understanding AGN fueling.

Contribution

First detailed kinematic profiles of active galaxy nuclei using near-infrared spectroscopy, identifying decoupled nuclear structures and transient discs.

Findings

Nuclear bars show decoupled kinematics with velocity peaks in the nucleus.

Significant central velocity dispersion drops observed in three galaxies.

Dynamical models confirm decoupled nuclear components but not the dispersion drops.

Abstract

We present new stellar kinematical profiles of four galaxy hosts of active galactic nuclei, using the CO bandhead around 2.3 micron 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 (m=1 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.