Broad He I 1.08 {\mu}m absorption from the obscurer in the active galaxy NGC 5548

TL;DR

This study analyzes He I 1.08 μm absorption in NGC 5548, revealing variability linked to the obscurer and warm absorber, supporting a model where helium recombination influences the active galaxy's gas dynamics.

Contribution

It provides a detailed decomposition and variability analysis of He I absorption, supporting a physical cycle model of obscurer-warm absorber interaction in NGC 5548.

Findings

Detection of fast, broad He I absorption linked to the obscurer

Variability of absorption consistent with ionization changes

Support for the helium recombination cycle model

Abstract

The nucleus of the active galaxy NGC 5548 was the target of two intensive spectroscopic monitoring campaigns at X-ray, ultraviolet (UV), and optical frequencies in 2013/14. These campaigns detected the presence of a massive obscuration event. In 2016/17, Landt et al. conducted a near-IR spectroscopic monitoring campaign on NGC 5548 and discovered He i 1.08 {\mu}m absorption. Here we decompose this absorption into its components and study its time variability. We attribute the narrow He i absorption lines to the warm absorber and, as for the newly appeared low-ionization warm absorber lines in the UV, their presence is most likely due to a reduction in ionization parameter caused by the obscurer. The observed variability of the narrow He i absorption is consistent with what is expected for the warm absorber. Most importantly, we also detect fast, broad He i absorption, which we attribute to the obscurer. This He i broad absorption, which is indicative of a high-column density gas, is unsaturated and variable on time-scales of a few months. The observed variability of the obscurer is mainly due to changes in ionization, although density changes also play a role. We test the physical cycle model of Dehghanian et al. which proposes that helium recombination can account for how the obscurer influences the physics of the warm absorber gas. Our results support their model, but also indicate that the reality might be more complex.