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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.