The faint source population at 15.7 GHz - III. A high-frequency study of
  HERGs and LERGs
  A complete sample of 96 faint ($S > 0.5$ mJy) radio galaxies is selected from
the Tenth Cambridge (10C) survey at 15.7~GHz. Optical spectra are used to
classify 17 of the sources as high-excitation or low-excitation radio galaxies
(HERGs and LERGs respectively), for the remaining sources three other methods
are used; these are optical compactness, X-ray observations and mid-infrared
colour--colour diagrams. 32 sources are HERGs and 35 are LERGs while the
remaining 29 sources could not be classified. We find that the 10C HERGs tend
to have higher 15.7-GHz flux densities, flatter spectra, smaller linear sizes
and be found at higher redshifts than the LERGs. This suggests that the 10C
HERGs are more core dominated than the LERGs.
  Lower-frequency radio images, linear sizes and spectral indices are used to
classify the sources according to their radio morphology; 18 are Fanaroff and
Riley type I or II sources, a further 13 show some extended emission, and the
remaining 65 sources are compact and are referred to as FR0 sources. The FR0
sources are sub-divided into compact, steep-spectrum (CSS) sources (13 sources)
or GHz-peaked spectrum (GPS) sources (10 sources) with the remaining 42 in an
unclassified class. FR0 sources are more dominant in the subset of sources with
15.7-GHz flux densities $<$1 mJy, consistent with the previous result that the
fainter 10C sources have flatter radio spectra.
  The properties of the 10C sources are compared to the higher-flux density
Australia Telescope 20 GHz (AT20G) survey. The 10C sources are found at similar
redshifts to the AT20G sources but have lower luminosities. The nature of the
high-frequency selected objects change as flux density decreases; at high flux
densities the objects are primarily quasars, while at low flux densities radio
galaxies dominate.
