SpitzerObservations of 3C Quasars and Radio Galaxies: Mid‐Infrared Properties of Powerful Radio Sources

We have measured mid-infrared radiation from an orientation-unbiased sampleof 3CRR galaxies and quasars at redshifts 0.4 < z < 1.2 with the IRS and MIPSinstruments on the Spitzer Space Telescope. Powerful emission (L_24micron >10^22.4 W/Hz/sr) was detected from all but one of the sources. We fit theSpitzer data as well as other measurements from the literature with synchrotronand dust components. The IRS data provide powerful constraints on the fits. At15 microns, quasars are typically four times brighter than radio galaxies withthe same isotropic radio power. Based on our fits, half of this difference canbe attributed to the presence of non-thermal emission in the quasars but notthe radio galaxies. The other half is consistent with dust absorption in theradio galaxies but not the quasars. Fitted optical depths are anti-correlatedwith core dominance, from which we infer an equatorial distribution of dustaround the central engine. The median optical depth at 9.7 microns for objectswith core-dominance factor R > 10^-2 is approximately 0.4; for objects with R <10^-2, it is 1.1. We have thus addressed a long-standing question in theunification of FR II quasars and galaxies: quasars are more luminous in themid-infrared than galaxies because of a combination of Doppler-boostedsynchrotron emission in quasars and extinction in galaxies, bothorientation-dependent effects.Comment: 42 pages, 14 figures plus two landscape tables. Accepted for publication in Ap