Physical properties and the variability mechanism of the He i outflow in NGC 4151

We report on variable helium absorption lines in NGC 4151 observed across six epochs of quasi-simultaneous near-infrared and optical data. These observations cover the transitions from the metastable 23S state at 3889 Å and 10 830 Å, and from the 21S state at 20 587 Å. This is the first active galactic nuclei absorption line variability study to include measurements of the 20 587 Å line. The physi...