The Correlation Between [O IV] λ25.89µ and the Hard X-ray Luminosities for a Sample of Nearby Seyfert Galaxies

We have studied the relationship between the [O IV] λ25.89µm emission line luminosities, obtained from Spitzer spectra,

with the X-ray continua in the 2-10 keV band, primarily from ASCA, and the 14-195 keV band obtained with the SWIFT/Burst Alert Telescope (BAT), for a sample of nearby (z < 0.08) Seyfert galaxies.

For comparison, we have examined the relationship between the [O III] λ5007, the 2-10 keV and the 14-195 keV luminosities for the same set of objects. We find that both the [O IV] and [O III] luminosities are well-correlated with the BAT luminosities. On the other hand, the [O III] luminosities are better-correlated with 2-10 keV luminosities than are those of [O III]. When comparing [O IV] and [O III] luminosities for the different types of galaxies, we find that the Type 2's have significantly lower [O III] to [O IV] ratios than the Type 1's.

The combined effects of reddening and the X-ray absorption are the probable reason that the [O III] - 2-10 keV correlation is better than than that for the [O IV] - 2-10 keV, since the [O IV] λ25.89µm emission line is much less affected by extinction.

We present a grid of photoionization models used to calculate the physical conditions present in the [O IV] region. We find that the [O IV] comes from higher ionization states and lower density regions than previous studies had determined for [O III].

Overall, we find the [O IV] to be an accurate and truly isotropic indicator of the power of the AGN. This suggests that it can be useful in deconvolving the contributions of the AGN and starburst in the spectra of Compton-thick and/or X-ray weak sources.