The Mid-Infrared High-Ionization Lines from Active Galactic Nuclei and Star-Forming Galaxies

TL;DR

This study analyzes mid-infrared emission lines in 426 galaxies, revealing correlations among high-ionization lines and distinguishing contributions from active galactic nuclei and star formation.

Contribution

It provides new insights into the relationships among mid-IR emission lines and quantifies the contributions of AGN and star formation to these lines.

Findings

Strong linear correlation between [Ne V] and [O IV] lines.

AGN dominate [O IV] emission in most Seyferts.

Star formation explains [O IV] in HII galaxies.

Abstract

We used Spitzer/IRS spectroscopic data on 426 galaxies including quasars, Seyferts, LINER and HII galaxies to investigate the relationship among the mid-IR emission lines. There is a tight linear correlation between the [Ne V]14.3 um and 24.3 um (97.1 eV) and the [O IV]25.9 um (54.9 eV) high-ionization emission lines. The correlation also holds for these high-ionization emission lines and the [Ne III]15.56 um (41 eV) emission line, although only for active galaxies. We used these correlations to calculate the [Ne III] excess due to star formation in Seyfert galaxies. We also estimated the [O IV] luminosity due to star formation in active galaxies and determined that it dominates the [O IV] emission only if the contribution of the active nucleus to the total luminosity is below 5%. We find that the AGN dominates the [O IV] emission in most Seyfert galaxies, whereas star-formation adequately explains the observed [O IV] emission in optically classified HII galaxies. Finally we computed photoionization models to determine the physical conditions of the narrow line region where these high-ionization lines originate. The estimated ionization parameter range is -2.8 < log U < -2.5 and the total hydrogen column density range is 20 < log nH (cm-2) < 21.