New Indicators for AGN Power: The Correlation between [O IV] 25.89 μm and Hard X-Ray Luminosity for Nearby Seyfert Galaxies

We have studied the relationship between the [O IV] 25.89 μm emission-line luminosities, obtained from Spitzer spectra, 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. When comparing [O IV] and [O III] luminosities for the different types of galaxies, we find that the Seyfert 2s have significantly lower [O III] to [O IV] ratios than the Seyfert 1s. We suggest that this is due to more reddening of the narrow-line region (NLR) of the Seyfert 2s, since the [O IV] 25.89 μm emission line is much less affected by extinction. The combined effects of reddening and the X-ray absorption is the probable reason why the [O III] versus 2-10 keV correlation is better than the [O IV] versus 2-10 kev correlation. Based on photoionization models, 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 indicator of the power of the AGN.

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