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

Abstract

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 morphology of Cygnus A, where all of the mid-IR emission is consistent with reprocessing by the hidden quasar known to exist from spectropolarimetry and other evidence.