Gas and stellar dynamics in NGC 1068: probing the galactic gravitational potential

We present SAURON integral field spectrography of the central 1.5 kpc of the nearby Seyfert 2 galaxy NGC 1068, encompassing the well-known near-infrared (NIR) inner bar observed in the K band. We have successively disentangled the respective contributions of the ionized gas and stars, thus deriving their two-dimensional distribution and kinematics. The [O III] and H beta emission lines exhibit a very different spatial distribution and kinematics, the latter following inner spiral arms with clumps associated with star formation. Strong inward streaming motions are observed in both the H beta and [O III] kinematics. The stellar kinematics also exhibit clear signatures of a non-axisymmetric tumbling potential, with a twist in both the velocity and Gauss-Hermite h(3) fields. We re-examined the long-slit data of Shapiro, Gerssen & van der Marel using a pPXF: a strong decoupling of the Gauss-Hermite term h(3) is revealed, and the central decrease of Gauss-Hermite term h(4) hinted in the SAURON data is confirmed. These data also suggest that NGC 1068 is a good candidate for a so-called sigma drop. We confirm the possible presence of two separate pattern speeds applying the Tremaine-Weinberg method to the Fabry-Perot H alpha map. We also examine the stellar kinematics of bars formed in N-body + smoothed particle hydrodynamics (SPH) simulations built from axisymmetric initial conditions approximating the luminosity distribution of NGC 1068. The resulting velocity, dispersion and higher order Gauss-Hermite moments successfully reproduce a number of properties observed in the two-dimensional kinematics of NGC 1068 and the long-slit data, showing that the kinematic signature of the NIR bar is imprinted in the stellar kinematics. The remaining differences between the models and the observed properties are likely mostly due to the exclusion of star formation and the lack of the primary large-scale oval/bar in the simulations. These models nevertheless suggest that the inner bar could drive a significant amount of gas down to a scale of similar to 300 pc. This would be consistent with the interpretation of the sigma drop in NGC 1068 being the result of central gas accretion followed by an episode of star formation.