SpitzerReveals Hidden Quasar Nuclei in Some Powerful FR II Radio Galaxies

We present a Spitzer mid-infrared survey of 42 Fanaroff-Riley class II radiogalaxies and quasars from the 3CRR catalog at redshift z<1. All of the quasarsand 45+/-12% of the narrow-line radio galaxies have a mid-IR luminosity ofnuLnu(15 micron) > 8E43 erg/s, indicating strong thermal emission from hot dustin the active galactic nucleus. Our results demonstrate the power of Spitzer tounveil dust-obscured quasars. The ratio of mid-IR luminous narrow-line radiogalaxies to quasars indicates a mean dust covering fraction of 0.56+/-0.15,assuming relatively isotropic emission. We analyze Spitzer spectra of the 14mid-IR luminous narrow-line radio galaxies thought to host hidden quasarnuclei. Dust temperatures of 210-660 K are estimated from single-temperatureblackbody fits to the low and high-frequency ends of the mid-IR bump. Most ofthe mid-IR luminous radio galaxies have a 9.7 micron silicate absorption troughwith optical depth <0.2, attributed to dust in a molecular torus. Forbiddenemission lines from high-ionization oxygen, neon, and sulfur indicate a sourceof far-UV photons in the hidden nucleus. However, we find that the other55+/-13% of narrow-line FR II radio galaxies are weak at 15 micron, contrary tosingle-population unification schemes. Most of these galaxies are also weak at30 micron. Mid-IR weak radio galaxies may constitute a separate population ofnonthermal, jet-dominated sources with low accretion powerComment: 34 pages, 8 figures, ApJ submitte