Searching for Nuclear Obscuration in the Infrared Spectra of Nearby FR I Radio Galaxies

TL;DR

This study investigates infrared spectra of nearby FR I radio galaxies to detect signs of nuclear obscuration by dust, finding that a minority show evidence consistent with a clumpy torus structure.

Contribution

It provides the first wide-band infrared spectroscopic analysis of FR I galaxies to assess the presence of dusty obscuring structures, using advanced spectral decomposition techniques.

Findings

40% of galaxies show evidence of obscuring dust.

10% of galaxies are consistent with a clumpy torus model.

Some galaxies have thermal mid-infrared components indicating dust heating.

Abstract

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.