Gemini/GMOS IFU gas velocity 'tomography' of the narrow line region of nearby active galaxies

Abstract

We present two-dimensional (2D) mapping of the gas velocity field of the inner few hundred parsecs of six nearby active galaxies, using spectra obtained with the integral field unit of the Gemini Multi-Object Spectrograph instrument at the Gemini North telescope. In our previous paper, we reported the 2D mapping of the stellar kinematics extracted from the calcium triplet absorption lines. In this paper, we use the [S III] λ9069 emission line to obtain the flux distribution and kinematics of the gas in the narrow-line region (NLR). The gas emission is extended by a few hundred parsecs and its kinematics are dominated by rotation in the galaxy plane. Subtraction of the rotation component reveals outflows along the NLR which show spatial correlation with radio structures seen in Very Large Array radio 3.6 and 20 cm flux images, suggesting that the radio jet is pushing the circumnuclear interstellar medium. This interpretation is also supported by the observation of high-velocity dispersion (σ ≥ 500 km s-ˡ) structures in association with the outflowing gas. The gas outflows and radio jets are oriented at random angles relative to the galaxy major axis, indicating that they are not launched perpendicularly to the galaxy plane. Slicing the emission-line profiles into velocity channels, we create maps of the NLR gas distribution at different radial velocities. In at least half of our sample, the highest velocities are observed close to the nucleus suggesting that the emitting gas is decelerating outwards, from projected blueshifts exceeding 400 km s-ˡ to values of 100–200 km s-ˡ at 100–200 pc from the nucleus. We have estimated mass outflow rates in the NLR of ≈1 to 50 × 10-³Mʘ yr-ˡ, which are ≈10–20 times the accretion rate necessary to feed the active nucleus. The kinetic energy of the ouflow is estimated to be 4–5 orders of magnitude smaller than the bolometric luminosity. Assuming kinetic energy transfer between the radio jet and the NLR outflows, the mass ejection rate in the radio jet is 5–6 orders of magnitude smaller than the mass accretion rate necessary to feed the nuclear supermassive black hole.