Kinematic Analysis of Nuclear Spirals: Feeding the Black Hole in Ngc1097

We present a harmonic expansion of the observed line-of-sight velocity field as a method to recover and investigate spiral structures in the nuclear regions of galaxies. Application to the emission-line velocity field within the circumnuclear starforming ring of NGC1097, obtained with the GMOS-IFU spectrograph, reveals a three-arm spiral in the non-circular motions, which agrees with a two-arm dust spiral in the surface brightness. This nuclear spiral is consistent with a weak perturbation in the gravitational potential due to a two-arm logarithmic spiral, with a pitch angle of 52± 4 derived directly from the harmonic expansion of the velocity field. Next, we use a simple spiral perturbation model to constrain the fraction of the measured non-circular motions that is due to radial inflow. We combine the resulting inflow velocity with the gas density in the spiral arms, inferred from emission line ratios, to estimate the mass inflow rate as a function of radius, which reaches about 0.011M⊙ yr −1 at a distance of 70 pc from the center. This corresponds to a fraction of about 4.2 × 10 of the Eddington mass accretion rate onto the central black hole in NGC1097, and is fully consistent with previous mass accretion models fitted to the observed spectral energy distribution in the nucleus of this LINER/Seyfert1 galaxy. We conclude that the line-of-sight velocity not only can provide a cleaner view of nuclear spirals than the associated dust, but that the presented method also allows the quantitative study of these possibly important links in fueling the centers of galaxies, including providing a handle on the mass inflow rate as a function of radius. Subject headings: galaxies: active — galaxies: kinematics and dynamics — galaxies: nuclei — galaxies: structure — galaxies: individual: NGC1097