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

TL;DR

This study investigates the ionization structure of the Narrow Line Region in NGC 4151 using Hubble imaging and photoionization models, revealing filtering effects by ionized absorbers consistent with disk-wind models.

Contribution

It provides new insights into the ionization filtering processes in the NLR of NGC 4151, supporting the disk-wind model with detailed imaging and modeling analysis.

Findings

[O III]/[O II] ratios indicate ionization filtering effects.

Ionized absorbers influence the NLR structure.

Results support disk-wind models for NLR formation.

Abstract

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 galaxy NGC 4151. While the NLR of NGC 4151 exhibits an overall structure consistent with 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 Hubble Space Telescope reveal significant emission from outside the the emission-line bi-cone. The [O III]/[O II] ratios are lower in these regions, consistent with a weaker ionizing flux. We performed a photoionization modeling analysis of the emission-line gas within a series of annuli, centered on the the central continuum source, with inner radii from 13 to 90 pc. The gas is ionized by radiation that has been attenuated by a relatively highly-ionized absorber (HABS), which completely covers the central source, and a lower-ionization absorber (LABS), which has a covering factor ranging from 0 to 1. We found that the [O III]/[O II] ratios are well fit by assuming that, within each segment of an annulus, some fraction of the NLR gas is completely within the shadow of LABS, while the rest is irradiated by the continuum filtered only by HABS. This suggests that the structure of the NLR is due to filtering of the ionizing radiation by ionized gas, consistent with disk-wind models. One possible scenario is that the low-ionization absorbers are dense knots of gas swept up by a wind.