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We present a study of the distribution of [O III] $\lambda$5007 and [O II]$\lambda$3727 emission in the Narrow Line Region (NLR) of the Seyfert 1 galaxyNGC 4151. While the NLR of NGC 4151 exhibits an overall structure consistentwith the unified model of Seyfert galaxies, narrow-band [O III] and [O II]images obtained with the Wide Field and Planetary Camera 2 aboard the HubbleSpace Telescope reveal significant emission from outside the the emission-linebi-cone. The [O III]/[O II] ratios are lower in these regions, consistent witha weaker ionizing flux. We performed a photoionization modeling analysis of theemission-line gas within a series of annuli, centered on the the centralcontinuum source, with inner radii from 13 to 90 pc. The gas is ionized byradiation that has been attenuated by a relatively highly-ionized absorber(HABS), which completely covers the central source, and a lower-ionizationabsorber (LABS), which has a covering factor ranging from 0 to 1. We found thatthe [O III]/[O II] ratios are well fit by assuming that, within each segment ofan annulus, some fraction of the NLR gas is completely within the shadow ofLABS, while the rest is irradiated by the continuum filtered only by HABS. Thissuggests that the structure of the NLR is due to filtering of the ionizingradiation by ionized gas, consistent with disk-wind models. One possiblescenario is that the low-ionization absorbers are dense knots of gas swept upby a wind.

Probing the Ionization Structure of the Narrow Line Region in the Seyfert 1 Galaxy NGC 4151