Near-infrared spectroscopy of nearby Seyfert galaxies: Is there evidence for the shock excitation in narrow-line regions?

TL;DR

This study investigates the ionization mechanisms in Seyfert galaxy narrow-line regions using near-infrared spectroscopy, finding evidence for shock excitation in some cases and photoionization in others, with no clear link to radio jets.

Contribution

It provides new spectroscopic measurements of [Fe II]/[P II] flux ratios in 26 Seyfert galaxies, combining literature data to assess shock versus photoionization contributions.

Findings

Three galaxies show high [Fe II]/[P II] ratios indicating shock excitation.

Most galaxies have ratios consistent with photoionization.

No correlation between flux ratio and radio loudness.

Abstract

One of the unsettled, important problems in active galactic nuclei (AGNs) is the major ionization mechanism of gas clouds in their narrow-line regions (NLRs). In order to investigate this issue, we present our $J$-band spectroscopic observations for a sample of 26 nearby Seyfert galaxies. In our study, we use the flux ratio of the following two forbidden emission lines, [Fe II]1.257$\mu$m and [P II]1.188$\mu$m because it is known that this ratio is sensitive to the ionization mechanism. We obtain the [Fe II]/[P II] flux ratio or its lower limit for 19 objects. In addition to our data, we compile this flux ratio (or its lower limit) of 23 nearby Seyfert galaxies from the literature. Based on the collected data, we find that three Seyfert galaxies show very large lower limits of [Fe II]/[P II] flux ratios ($\gtrsim$10); NGC 2782, 5005, and Mrk 463. It is thus suggested that the contribution of the fast shock in the gas excitation is significantly large for them. However, more than half of Seyfert galaxies in our sample show moderate [Fe II]/[P II] flux ratios ($\sim$2), that is consistent to the pure photoionization by power-law ionizing continuum emission. We also find that the [Fe II]/[P II] flux ratio shows no clear correlation with the radio loudness, suggesting that the radio jet is not the primary origin of shocks in NLRs of Seyfert galaxies.