  Radio galaxies were discovered and mapped in the 1950s. The optical spectra
showed little or no nuclear continuum light. Some also revealed powerful high
ionization emission lines, while others showed at most weak low-ionization
emission lines. Quasars were found in the 1960s, and their spectra were
dominated by powerful continuum radiation which was subsequently identified
with optically thick thermal radiation from copious accretion flows, as well as
high ionization narrow emission lines, and powerful broad permitted lines. By
the 1980s, data from optical polarization and statistics of the radio
properties required that many radio galaxies contain hidden quasar nuclei,
hidden from the line of sight by dusty, roughly toroidal gas distributions. The
radio galaxies with hidden quasars are referred to as "thermal." Do all radio
galaxies have powerful hidden quasars? We now know the answer using arguments
based on radio, infrared, optical and X-ray properties. Near the top of the
radio luminosity function, for FRII, GPS, and CSS galaxies, the answer is yes.
Below the top of the radio luminosity function, many do not. At low radio
luminosities, most do not. Instead these "nonthermal" weakly-accreting galaxies
are powerful emitters of kinetic energy in the form of synchrotron jets. They
generally have weak low-ionization lines. This applies to all types of radio
galaxy, big FR II doubles, as well as the small young GigaHertz-Peaked-Spectrum
and Compact Steep Spectrum sources. Only a few FR I sources are of the thermal
type.
