Spitzer Reveals Hidden Quasar Nuclei in Some Powerful FR II Radio Galaxies

Details Spitzer Reveals Hidden Quasar Nuclei in Some Powerful FR II Radio Galaxies Spitzer Reveals Hidden Quasar Nuclei in Some Powerful FR II Radio Galaxies

2006-01-20

arxiv.org/abs/astro-ph/0601485v1

Astrophys.J.647:161-171,2006

Astronomy and Astrophysics

Astrophysics

34 pages, 8 figures, ApJ submitted

Scientific paper

10.1086/505337

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 nuLnu(15 micron) > 8E43 erg/s, 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 micron 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 micron, contrary to single-population unification schemes. Most of these galaxies are also weak at 30 micron. Mid-IR weak radio galaxies may constitute a separate population of nonthermal, jet-dominated sources with low accretion power

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