The first near-infrared, high-resolution échelle spectroscopy of the outflow in NGC 4151

A study of the clouds covering the Eye of Sauron

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone (RM), Italy
² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
³ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁴ ASI – Space Science Data Center, Via del Politecnico snc, I-00133 Roma, Italy
⁵ Dip. di Matematica e Fisica, Università degli Studi “Roma Tre”, Via della Vasca Navale 84, I-00146 Roma, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 October 2025
Accepted: 24 May 2026

Abstract

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 Iλ10 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 (λ/Δλ > 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 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 – Ṁ_out ≲ 10⁻¹ M_⊙ yr⁻¹, r_out ∼ 3 pc, v_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 (Ė_kin/L_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.