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