The first near-infrared, high-resolution échelle spectroscopy of the outflow in NGC 4151. A study of the clouds covering the Eye of Sauron

We present the first high-resolution near-infrared spectroscopy of the nucleus of the nearby, well-known Seyfert galaxy NGC 4151 (the `Eye of Sauron'). Past studies of this source have revealed that it exhibits a variable absorption feature associated with the He I łambda10,830 Å emission line, that is potentially indicative of obscuration events affecting the central engine. Here, we took advantage of the IRTF /iSHELL and TNG /GIANO-B spectrographs to observe this feature with unprecedented spectral resolution (łambda/ absorption variability and that of the X-ray ionising continuum, we also analysed the publicly available data collected by the > 50,000); these allowed us to study the structure of the absorption trough and its variations in detail over a time span of ∼700 days. In order to infer a connection between the He I Swift -XRT instrument over the same period of time, unveiling a potential driving mechanism in the changes of the outflow ionisation state due to the X-ray flux variations. We also derive outflow physical parameters---dot M _ ̊m out łesssim 10^ -2 M_⊙ yr -1 $, r_ ̊m out ∼ 3 pc, v_ ̊m max ∼ 1000 km s^-1---that are in line with those of comparable ionised winds found in similar targets, where the outflow is not powerful enough to trigger a significant feedback from the active galactic nucleus ($ _ E ̊m kin /L_ ̊m bol ∼ 0.001%). Such findings point at a scenario in which a dusty and clumpy outflow that obscures NGC 4151 up to galactic scales responds to changes in the ionising X-ray flux; this is similar to what happens in quasars with broad absorption lines and Seyferts with multi-phase outflows such as NGC 5548.