Spitzer Mid-IR Spectroscopy of Powerful 2Jy and 3CRR Radio Galaxies. II. AGN Power Indicators and Unification

TL;DR

This study evaluates various mid-infrared and other wavelength diagnostics to identify the most reliable indicators of AGN bolometric power in powerful radio galaxies, emphasizing the effectiveness of the [O IV] line.

Contribution

It demonstrates that the mid-IR [O IV] line is the most reliable AGN power indicator and compares its effectiveness against other diagnostics like [O III], 24 μm, radio, and X-ray emissions.

Findings

[O IV] line is the most reliable AGN power indicator.

[O III] and 24 μm emissions are mildly attenuated but better indicators than radio or X-ray.

Many objects show no clear 9.7 μm silicate feature, indicating variability in dust properties.

Abstract

It remains uncertain which continuum and emission line diagnostics best indicate the bolometric powers of active galactic nuclei (AGNs), especially given the attenuation caused by the circumnuclear material and the possible contamination by components related to star formation. Here we use mid-IR spectra along with multiwavelength data to investigate the merit of various diagnostics of AGN radiative power, including the mid-IR [Ne III] lambda25.89 mum and [O IV] lambda25.89 mum fine-structure lines, the optical [O III] lambda5007 forbidden line, and mid-IR 24 mum, 5 GHz radio, and X-ray continuum emission, for complete samples of 46 2Jy radio galaxies (0.05 < z < 0.7) and 17 3CRR FRII radio galaxies (z < 0.1). We find that the mid-IR [O IV] line is the most reliable indicator of AGN power for powerful radio-loud AGNs. By assuming that the [O IV] is emitted isotropically, and comparing the [O III] and 24 mum luminosities of the broad- and narrow-line AGNs in our samples at fixed [O IV] luminosity, we show that the [O III] and 24 mum emission are both mildly attenuated in the narrow-line compared to the broad-line objects by a factor of #2. However, despite this attenuation, the [O III] and 24 mum luminosities are better AGN power indicators for our sample than either the 5 GHz radio or the X-ray continuum luminosities. We also detect the mid-IR 9.7 mum silicate feature in the spectra of many objects but not ubiquitously: at least 40% of the sample shows no clear evidence for these features. We conclude that, for the majority of powerful radio galaxies, the mid-IR lines are powered by AGN photoionization.