Feeding versus feedback in NGC4151 probed with Gemini NIFS – I. Excitation

Abstract

We have used the Gemini Near-infrared Integral Field Spectrograph (NIFS) to map the emission-line intensity distributions and ratios in the narrow-line region (NLR) of the Seyfert galaxy NGC4151 in the Z, J, H and K bands at a resolving power ≥5000, covering the inner ≈200 × 300 pc of the galaxy at a spatial resolution of ≈8 pc. We present intensity distributions in 14 emission lines, which show three distinct behaviours. (1) Most of the ionized gas intensity distributions are extended to ≈100 pc from the nucleus along the region covered by the known biconical outflow (position angle, PA = 60/240◦, NE–SW), consistent with an origin in the outflow; while the recombination lines show intensity profiles which decrease with distance r from the nucleus as I ∝ r-ˡ, most of the forbidden lines present a flat intensity profile (I ∝ rº) or even increasing with distance from the nucleus towards the border of the NLR. (2) The H2 emission lines show completely distinct intensity distributions, which avoid the region of the bicone, extending from ≈10 to ≈60 pc from the nucleus approximately along the large-scale bar, almost perpendicular to the bicone axis. This morphology supports an origin for the H2-emitting gas in the galaxy plane. (3) The coronal lines show a steep intensity profile, described by I ∝ r-²; the emission is clearly resolved only in the case of [Si VII], consistent with an origin in the inner NLR. Using the line-ratio maps [Fe II] 1.644/1.257 and Pa β/Br γ , we obtain an average reddening of E(B − V) ≈ 0.5 along the NLR and E(B − V) ≥ 1 at the nucleus. Our line-ratio map [Fe II] 1.257 μm/[P II] 1.189 μm of the NLR of NGC4151 is the first such map of an extragalactic source. Together with the [Fe II]/Paβ map, these line ratios correlate with the radio intensity distribution, mapping the effects of shocks produced by the radio jet on the NLR. These shocks probably release the Fe locked in grains and produce an enhancement of the [Fe II] emission at ≈1 arcsec from the nucleus. At these regions, we obtain electron densities Ne ≈ 4000 cm-³ and temperatures Te ≈ 15 000K for the [Fe II]-emitting gas. For the H2-emitting gas, we obtain much lower temperatures of Texc ≈ 2100K and conclude that the gas is in thermal equilibrium. The heating necessary to excite the molecule may be due to X-rays escaping perpendicular to the cone (through the nuclear torus, if there is one) or to shocks probably produced by the accretion flow previously observed along the large-scale bar. The distinct intensity distributions and physical properties of the ionized and molecular gas, as well as their locations, the former along the outflowing cone, and the latter in the galaxy plane surrounding the nucleus, suggest that the H2-emitting gas traces the active galactic nuclei feeding, while the ionized gas traces its feedback.