Mid-infrared Spectra of Radio Galaxies and Quasars

Spitzer Infrared Spectrograph (IRS) observations of 3C radio galaxies and quasars shed new light on the nature of the central engines of AGN. Emission from silicate dust obscuring the central engine can be used to estimate the bolometric luminosity of an AGN. Emission lines from ions such as O IV and Ne V give another indication of the presence or lack of a hidden source of far-UV photons in the nucleus. Radio-loud AGN with relative-to-Eddington luminosity ratios of L/L_(Edd) < 3 x 10^(−3) do not appear to have broad optical emission lines, though some do have strong silicate emission. Aromatic emission features from star formation activity are common in low-luminosity radio galaxies. Strong H2 pure-rotational emission lines are also seen in some mid-IR-weak radio galaxies, caused by either merger shocks or jet shocks in the interstellar medium.