  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.
