Feeding versus feedback in NGC4151 probed with Gemini NIFS – II. Kinematics

Abstract

We have used the Gemini Near-infrared Integral Field Spectrograph (NIFS) to map the gas kinematics of the inner ~200 × 500 pc 2 of the Seyfert galaxy NGC4151 in the Z, J, H and K bands at a resolving power of ≥5000 and spatial resolution of ~8 pc. The ionized gas emission is most extended along the known ionization bi-cone at position angle PA = 60◦–240◦, but is observed also along its equatorial plane. This indicates that the active galactic nucleus (AGN) ionizes gas beyond the borders of the bi-cone, within a sphere with ≈1 arcsec radius around the nucleus. The ionized gas has three kinematic components: (1) one observed at the systemic velocity and interpreted as originating in the galactic disc; (2) one outflowing along the bicone, with line-of-sight velocities between −600 and 600 km s-ˡ and strongest emission at ±(100–300) km s-ˡ; and (3) another component due to the interaction of the radio jet with ambient gas. The radio jet (at PA = 75◦–255◦) is not aligned with the narrow-line region (NLR) and produces flux enhancements mostly observed at the systemic velocity, suggesting that the jet is launched close to the plane of the galaxy (approximately plane of the sky). The mass outflow rate, estimated to be ≈1Mʘ yr-ˡ along each cone, exceeds the inferred black hole accretion rate by a factor of ~100. This can be understood if the NLR is formed mostly by entrained gas from the circumnuclear interstellar medium by an outflow probably originating in the accretion disc. This flow represents feedback from the AGN, estimated to release a kinetic power of ˙E ≈ 2.4×10 41 erg s-ˡ,which is only ~0.3 per cent of the bolometric luminosity of the AGN. There is no evidence in our data for the gradual acceleration followed by gradual deceleration proposed by previous modelling of the [O III] emitting gas. Our data allow the possibility that the NLR clouds are accelerated close to the nucleus (within 0.1 arcsec, which corresponds to ≈7 pc at the galaxy) after which the flow moves at essentially constant velocity (≈600 km s-ˡ), being consistent with near-infrared emission arising predominantly from the interaction of the outflow with gas in the galactic disc. The molecular gas exhibits distinct kinematics relative to the ionized gas. Its emission arises in extended regions approximately perpendicular to the axis of the bi-cone and along the axis of the galaxy’s stellar bar, avoiding the innermost ionized regions. It does not show an outflowing component, being observed only at velocities very close to systemic, and is thus consistent with an origin in the galactic plane. This hot molecular gas may only be the tracer of a larger reservoir of colder gas which represents the AGN feeding.