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We report on variable helium absorption lines in NGC 4151 observed across sixepochs of quasi-simultaneous near-infrared and optical data. These observationscover the transitions from the metastable 2^3S state at 3889 A and 10830 A, andfrom the 2^1S state at 20587 A. This is the first AGN absorption linevariability study to include measurements of the 20587 A line. The physicalproperties of the absorber recorded at the fifth observational epoch arerelatively well constrained by the presence of absorption in both the opticaland near-infrared components, with the 10830 A line likely saturated. Theobservations suggest variations in this absorber's strength are best explainedby ionization changes in response to a variable incident continuum.Photoionization simulations constrain the total hydrogen number density of theepoch 5 absorber to 7.1<log(n_H/cm^-3)<8.8, the hydrogen column density to21.2<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 pcfrom the continuum emission region. This range in physical properties isconsistent with an absorber of similar velocity seen in NGC 4151 from previousultraviolet and optical studies, but with high column density X-ray absorbingcomponents not present. The mass outflow rate due to the fifth epoch absorberis in the range 0.008 to 0.38 M_sun/yr, too low to contribute to galaxyfeedback effects.

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