Thermal Emission as a Test for Hidden Nuclei in Nearby Radio Galaxies

The clear sign of a hidden quasar inside a radio galaxy is the appearance ofquasar spectral features in its polarized (scattered) light. However thatobservational test requires suitably placed scattering material to act as amirror, allowing us to see the nuclear light. A rather robust and more generaltest for a hidden quasar is to look for the predicted high mid-IR luminosityfrom the nuclear obscuring matter. The nuclear waste heat is detected and wellisolated in the nearest narrow line radio galaxy, Cen A. This confirms otherindications that Cen A does contain a modest quasar-like nucleus. However weshow here that M87 does not: at high spatial resolution, the mid-IR nucleus isseen to be very weak, and consistent with simple synchrotron emission from thebase of the radio jet. This fairly robustly establishes that there are "real"narrow line radio galaxies, without the putative accretion power, and withessentially all the luminosity in kinetic form. Next we show the intriguingmid-IR morphology of Cygnus A, reported previously by us and later discussed indetail by Radomski et al. (2002). All of this mid-IR emission is consistentwith reprocessing by a hidden quasar, known to exist from spectropolarimetry byOgle et al. (1997) and other evidence.Comment: 21 pages, 5 figure