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 2^3S state at 3889 A and 10830 A, and
from the 2^1S state at 20587 A. This is the first AGN absorption line
variability study to include measurements of the 20587 A line. The physical
properties of the absorber recorded at the fifth observational epoch are
relatively well constrained by the presence of absorption in both the optical
and near-infrared components, with the 10830 A line likely saturated. The
observations suggest variations in this absorber's strength are best explained
by ionization changes in response to a variable incident continuum.
Photoionization simulations constrain the total hydrogen number density of the
epoch 5 absorber to 7.1<log(n_H/cm^-3)<8.8, the hydrogen column density to
21.2<log(N_H/cm^-2)<23.3 and the ionization parameter range to -1.9<logU<0.4.
The simulations also suggest the absorber is located between 0.03 and 0.49 pc
from the continuum emission region. This range in physical properties is
consistent with an absorber of similar velocity seen in NGC 4151 from previous
ultraviolet and optical studies, but with high column density X-ray absorbing
components not present. The mass outflow rate due to the fifth epoch absorber
is in the range 0.008 to 0.38 M_sun/yr, too low to contribute to galaxy
feedback effects.
