We present a Spitzer mid-infrared survey of 42 Fanaroff-Riley class II radio galaxies and quasars from the 3CRR catalog at redshift z < 1. All of the quasars and 45 ± 12% of the narrow-line radio galaxies have a mid-IR luminosity of νL ν (15µm) > 8 × 10 43 erg s −1 , indicating strong thermal emission from hot dust in the active galactic nucleus. Our results demonstrate the power of Spitzer to unveil dust-obscured quasars. The ratio of mid-IR luminous narrow-line radio galaxies to quasars indicates a mean dust covering fraction of 0.56 ± 0.15, assuming relatively isotropic emission. We analyze Spitzer spectra of the 14 mid-IR luminous narrow-line radio galaxies thought to host hidden quasar nuclei. Dust temperatures of 210-660 K are estimated from single-temperature blackbody fits to the low and high-frequency ends of the mid-IR bump. Most of the mid-IR luminous radio galaxies have a 9.7 µm silicate absorption trough with optical depth < 0.2, attributed to dust in a molecular torus. Forbidden emission lines from high-ionization oxygen, neon, and sulfur indicate a source of far-UV photons in the hidden nucleus. However, we find that the other 55 ± 13% of narrow-line FR II radio galaxies are weak at 15 µm, contrary to single-population unification schemes. Most of these galaxies are also weak at 30 µm. Mid-IR weak radio galaxies may constitute a separate population of nonthermal, jet-dominated sources with low accretion power.

We present extinction curves derived from the broad emission lines and continua of samples of 72 radio-loud and 1018 radio-quiet AGNs. The curves are significantly flatter in the UV than are curves for the local ISM. The reddening curves for the radio-quiet LBQS quasars are slightly steeper than those of the radio-loud quasars in the UV, probably because of additional reddening by dust further out in the host galaxies of the former. The UV extinction curves for the radio-loud AGNs are very flat. This is explicable with slight modifications to standard MRN dust models: there is a relative lack of small grains in the nuclear dust. Our continuum and broad-emission line reddening curves agree in both shape and amplitude, confirming that the continuum shape is indeed profoundly affected by reddening for all but the bluest AGNs. With correction by our generic extinction curve, all of the radio-loud AGNs have continuous optical-UV spectra consistent with a single shape. We show that radio-quiet AGNs have very similar intrinsic UV to optical shape over orders of magnitude in luminosity. We also argue that radio-loud and radio-quiet AGNs probably share the same underlying continuum shape and that most of the systematic differences between their observed continuum shapes are due to higher nuclear reddening in radio-selected AGNs, and additional reddening from dust further out in the host galaxies in radio-quiet AGNs. Our conclusions have important implications for the modelling of quasar continua and the analysis of quasar demographics.

The clear sign of a hidden quasar inside a radio galaxy is the appearance of quasar spectral features in its polarized (scattered) light. However, that observational test requires suitably placed scattering material to act as a mirror, allowing us to see the nuclear light. A rather robust and more general test for a hidden quasar is to look for the predicted high mid-IR luminosity from the nuclear obscuring matter. The nuclear waste heat is detected and well isolated in the nearest narrow-line radio galaxy, Cen A. This confirms other indications that Cen A does contain a modest quasar-like nucleus. However, we show here that M87 does not: at high spatial resolution, the mid-IR nucleus is seen to be very weak and consistent with simple synchrotron emission from the base of the radio jet. This fairly robustly establishes that there are ''real'' narrow-line radio galaxies, without the putative accretion power and with essentially all the luminosity in kinetic form. Next, we show the intriguing morphology of Cygnus A, where all of the mid-IR emission is consistent with reprocessing by the hidden quasar known to exist from spectropolarimetry and other evidence.

The Spitzer spectrum of the giant FR II radio galaxy 3C 326 is dominated by very strong molecular hydrogen emission lines on a faint IR continuum. The H 2 emission originates in the northern component of a double-galaxy system associated with 3C 326. The integrated luminosity in H 2 pure rotational lines is 8:0 ; 10 41 erg s À1 , which corresponds to 17% of the 8Y70 m luminosity of the galaxy. A wide range of temperatures (125Y1000 K) is measured from the H 2 0Y0 S(0)YS (7) transitions, leading to a warm H 2 mass of 1:1 ; 10 9 M . Low-excitation ionic forbidden emission lines are consistent with an optical LINER classification for the active nucleus, which is not luminous enough to power the observed H 2 emission. The H 2 could be shock heated by the radio jets, but there is no direct indication of this. More likely, the H 2 is shock heated in a tidal accretion flow induced by interaction with the southern companion galaxy. The latter scenario is supported by an irregular morphology, a tidal bridge, and a possible tidal tail imaged with IRAC at 3Y9 m. Unlike ultraluminous infrared galaxies, which in some cases exhibit H 2 line luminosities of comparable strength, 3C 326 shows little star formation activity ($0.1 M yr À1 ). This may represent an important stage in galaxy evolution. Starburst activity and efficient accretion onto the central supermassive black hole may be delayed until the shock-heated H 2 can kinematically settle and cool.