Feeding versus feedback in NGC 1068 probed with Gemini NIFS – I. Excitation

Abstract

We present emission-line flux distributions and ratios for the inner ≈200 pc of the narrowline region (NLR) of the Seyfert 2 galaxy NGC 1068, using observations obtained with the Gemini Near-infrared Integral Field Spectrograph (NIFS) in the J, H and K bands at a spatial resolution of ≈10 pc and spectral resolution of ≈5300. The molecular gas emission – traced by the K-band H2 emission lines – outlines an off-centred circumnuclear ring with a radius of ≈100 pc showing thermal excitation. The ionized gas emission lines show flux distributions mostly outlining the previously known [O III] λ5007 ionization bi-cone. But while the flux distributions in the HI and He II emission lines are very similar to that observed in [OIII], the flux distribution in the [Fe II] emission lines is more extended and broader than a cone close to the nucleus, showing a ‘double bowl’ or ‘hourglass’ structure. This difference is attributed to the fact that the [Fe II] emission, besides coming from the fully ionized region, comes also from the more extended partially ionized regions, in gas excited mainly by X-rays from the active galactic nucleus. A contribution to the [Fe II] emission from shocks along the bi-cone axis to north-east and south-west of the nucleus is also supported by the enhancement of the [Fe II](1.2570 μm)/[P II](1.1885 μm) and [Fe II](1.2570 μm)/Paβ emission-line ratios at these locations and is attributed to the interaction of the radio jet with the NLR. The mass of ionized gas in the inner 200 pc of NGC 1068 is MHII ≈ 2.2 × 10⁴ Mʘ, while the mass of the H2 emitting gas is only MH₂ ≈ 29 Mʘ. Taking into account the dominant contribution of the cold molecular gas, we obtain an estimate of the total molecular gas mass of Mcold ≈ 2× 10⁷ Mʘ.