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