Searching for Nuclear Obscuration in the Infrared Spectra of Nearby FR I
  Radio Galaxies
  How do active galactic nuclei with low optical luminosities produce powerful
radio emission? Recent studies of active galactic nuclei with moderate radio
and low optical luminosities (Fanaroff & Riley class I, FR I) searching for
broad nuclear emission lines in polarized light, as predicted by some active
galactic nucleus unification models, have found heterogeneous results. These
models typically consist of a central engine surrounded by a torus of discrete
dusty clouds. These clouds would absorb and scatter optical emission, blocking
broad nuclear emission lines, and reradiate in mid-infrared. Some scattered
broad-line emission may be observable, depending on geometry, which would be
polarized. We present a wide-band infrared spectroscopic analysis of 10 nearby
FR I radio galaxies to determine whether there is significant emission from a
dusty obscuring structure. We used Markov Chain Monte Carlo algorithms to
decompose Spitzer/IRS spectra of our sample. We constrained the wide-band
behavior of our models with photometry from the Two Micron All Sky Survey,
Spitzer/IRAC, Spitzer/MIPS, and Herschel/SPIRE. We find that one galaxy is best
fit by a clumpy torus and three others show some thermal mid-infrared
component. This suggests that in those three there is likely some obscuring
dust structure that is inconsistent with our torus models and there must be
some source of photons heating the dust. We conclude that 40% of our FR I radio
galaxies show evidence of obscuring dusty material, possibly some other form of
hidden broad-line nucleus, but only 10% favor the clumpy torus model
specifically.
