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 H II galaxies to investigate the relationship among the mid-IR emission lines. There is a tight linear correlation between the [Ne v] 14.3 mu m and 24.3 mu m (97.1 eV) and the [O IV] 25.9 mu m (54.9 eV) high-ionization emission lines. The correlation also holds for these high-ionization emission lines and the [Ne III] 15.56 mu m (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 active galactic nucleus dominates the [O IV] emission in most Seyfert galaxies, whereas star formation adequately explains the observed [O IV] emission in optically classified H II 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.