Spectral Properties and the Effect on Redshift Cut-off of Compact AGNs
  from the AT20G Survey
  We use high angular resolution data, measured from visibility of sources at
the longest baseline of 4500 m of the Australia Telescope Compact Array (ATCA),
for the Australia Telescope 20 GHz (AT20G) survey to obtain angular size
information for > 94% of AT20G sources. We confirm the previous AT20G result
that due to the high survey frequency of 20 GHz, the source population is
strongly dominated by compact sources (79%). At 0.15 arcseconds angular
resolution limit, we show a very strong correlation between the compact and
extended sources with flat and steep-spectrum sources respectively. Thus, we
provide a firm physical basis for the traditional spectral classification into
flat and steep-spectrum sources to separate compact and extended sources. We
find the cut-off of -0.46 to be optimum for spectral indices between 1 and 5
GHz and, hence, recommend the continued use of -0.5 for future studies.
  We study the effect of spectral curvature on redshift cut-off of compact AGNs
using recently published redshift data. Using spectral indices at different
frequencies, we correct for the redshift effect and produce restframe frequency
spectra for compact sources for redshift up to ~5. We show that the flat
spectra of most compact sources start to steepen at ~30 GHz. At higher
frequencies, the spectra of both populations are steep so the use of spectral
index does not separate the compact and extended source populations as well as
in lower frequencies. We find that due to the spectral steepening, surveys of
compact sources at higher frequencies (>5 GHz) will have redshift cut-off due
to spectral curvature but at lower frequencies, the surveys are not
significantly affected by spectral curvature, thus, the evidence for a strong
redshift cut-off in AGNs found in lower frequency surveys is a real cut-off and
not a result of K-correction.
