BASS XLI: the correlation between Mid-infrared emission lines and Active Galactic Nuclei emission

Abstract

We analyze the Spitzer spectra of 140 active galactic nuclei (AGN) detected in the hard X-rays (14-195 keV) by the Burst Alert Telescope (BAT) on board Swift. This sample allows us to probe several orders of magnitude in black hole masses ($10^6-10^9{M}_{\odot }$), Eddington ratios ($10^{-3}-1$), X-ray luminosities ($10^{42}-10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$), and X-ray column densities ($10^{20}-10^{24}\mathrm{c}{\mathrm{m}}^{-2}$). The AGN emission is expected to be the dominant source of ionizing photons with energies $\gtrsim 50$ eV, and therefore high-ionization mid-infrared (MIR) emission lines such as [Ne V] 14.32, 24.32 $\hat{I}¼$m and [O IV] 25.89 $\hat{I}¼$m are predicted to be good proxies of AGN activity, and robust against obscuration effects. We find high detection rates ($\gtrsim 85-90$ per cent) for the mid-infrared coronal emission lines in our AGN sample. The luminosities of these lines are correlated with the 14-150 keV luminosity (with a typical scatter of $\ddot{I}ƒ\sim 0.4-0.5$ dex), strongly indicating that the mid-infrared coronal line emission is driven by AGN activity. Interestingly, we find that the coronal lines are more tightly correlated to the bolometric luminosity ($\ddot{I}ƒ\sim 0.2-0.3$ dex), calculated from careful analysis of the spectral energy distribution, than to the X-ray luminosity. We find that the relationship between the coronal line strengths and $L_{14-150\mathrm{keV}}$ is independent of black hole mass, Eddington ratio and X-ray column density. This confirms that the mid-infrared coronal lines can be used as unbiased tracers of the AGN power for X-ray luminosities in the $10^{42}-10^{45}\mathrm{erg}{\mathrm{s}}^{-1}$ range.

ApJ in press