The first near-infrared, high-resolution echelle 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 lambda 10 830 & Aring; 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 (lambda/Delta lambda > 50 000); these allowed us to study the structure of the absorption trough and its variations in detail over a time span of similar to 700 days. In order to infer a connection between the He I absorption variability and that of the X-ray ionising continuum, we also analysed the publicly available data collected by the 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 - M-out less than or similar to 10(-1) M-circle dot yr(-1), r(out) similar to 3 pc, v(max) similar to 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 (& Edot;(kin)/L-bol similar to 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.