The mid-infrared high-ionization lines from active galactic nuclei and star-forming galaxies

We used Spitzer/Infrared Spectrograph spectroscopic data on 426 galaxies including quasars, Seyferts, LINERs, 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 μm and 24.3 μm (97.1 eV) and the [Oiv]25.9 μm (54.9 eV) high-ionization emission lines. The correlation also holds for these high-ionization emission lines and the [Ne iii]15.56 μm (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 [Oiv] luminosity due to star formation in active galaxies and determined that it dominates the [Oiv] emission only if the contribution of the active nucleus to the total luminosity is below 5%. We find that the active galactic nucleus dominates the [Oiv] emission in most Seyfert galaxies, whereas star formation adequately explains the observed [Oiv] 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 n_H (cm-2) < 21.