Searching for nucleus obscuration in the infrared spectra of nearby FR-I radio galaxies

Abstract

Why are there so many types of active galactic nuclei (AGN)? Astronomers have proposed numerous AGN unification models to explain the vast array of different AGN types with a single type of object. The most prominent of these features a supermassive black hole’s accretion disk obscured by a torus of discrete dusty clouds. The applicability of this model to low-luminosity AGN such as Fanaroff & Riley class I (FR-I) radio galaxies is controversial. We present a wide-band infrared spectroscopic analysis of ten nearby FR-I radio galaxies. We used Markov-chain Monte Carlo algorithms to fit a set of models to Spitzer /IRS spectra with wide-band behaviour constrained by photometry from 2MASS, Spitzer /IRAC, Spitzer /MIPS, and Herschel /SPIRE. We find one galaxy is best fit by a torus and three others show some thermal mid-infrared component. We conclude that 40% of our FR-I radio galaxies show evidence of obscuring dusty material.