ar X iv : a st ro - p h / 02 07 38 5 v 1 1 8 Ju l 2 00 2 Thermal Emission as a Test for Hidden Nuclei in Nearby Radio

It is widely believed that the optical/UV continuum of quasars (the " Big Blue Bump ") represents optically thick thermal emission from accretion onto a black hole. Narrow line radio galaxies don't show such a component directly, and were historically thought for that reason to be rotation-powered, with large kinetic luminosity in the radio jets but very little accretion or optical radiation. When the Unified Model came along, identifying at least some narrow line radio galaxies as hidden quasars, the compelling observational motivation for this radio galaxy scenario lost some of its force. However, it is far from clear that all narrow line radio galaxies contain hidden quasar nuclei. 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 mid-IR morphology of Cygnus A, reported previously by us and later discussed in detail by Radomski et al. (2002). All of this mid-IR emission is consistent with reprocessing by a hidden quasar, known to exist from spectropolarimetry by Ogle et al. (1997) and other evidence.