Fig. 7.
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Evolution of He I column density over 20 years of IR spectroscopic observations of NGC 4151. Epochs before MJD = 58000 are taken from Wildy et al. (2016). Upper limits (black arrows) are computed at a 2σ confidence level. The average of the significant He I measurements (dot-dashed line), along with its 1σ uncertainty (dashed line), is indicated.
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