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 theappearance of quasar spectral features in its polarized (scattered)light. However, that observational test requires suitably placedscattering material to act as a mirror, allowing us to see the nuclearlight. A rather robust and more general test for a hidden quasar is tolook for the predicted high mid-IR luminosity from the nuclear obscuringmatter. The nuclear waste heat is detected and well isolated in thenearest narrow-line radio galaxy, Cen A. This confirms other indicationsthat Cen A does contain a modest quasar-like nucleus. However, we showhere that M87 does not: at high spatial resolution, the mid-IR nucleusis seen to be very weak and consistent with simple synchrotron emissionfrom the base of the radio jet. This fairly robustly establishes thatthere are ``real'' narrow-line radio galaxies, without the putativeaccretion power and with essentially all the luminosity in kinetic form.Next, we show the intriguing morphology of Cygnus A, where all of themid-IR emission is consistent with reprocessing by the hidden quasarknown to exist from spectropolarimetry and other evidence.