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ABSTRACT Our current understanding of radio-loud active galactic nuclei (AGN) comes predominantly from studies at frequencies of 5 GHz and below. With the recent completion of the Australia Telescope 20 GHz (AT20G) survey, we can now gain insight into the high-frequency radio properties of AGN. This paper presents supplementary information on the AT20G sources in the form of optical counterparts and redshifts. Optical counterparts were identified using the SuperCOSMOS data base and redshifts were found from either the 6dF Galaxy Survey or the literature. We also report 144 new redshifts. For AT20G sources outside the Galactic plane, 78.5 per cent have optical identifications and 30.9 per cent have redshift information. The optical identification rate also increases with increasing flux density. Targets which had optical spectra available were examined to obtain a spectral classification. There appear to be two distinct AT20G populations; the high luminosity quasars that are generally associated with point-source optical counterparts and exhibit strong emission lines in the optical spectrum, and the lower luminosity radio galaxies that are generally associated with passive galaxies in both the optical images and spectroscopic properties. It is suggested that these different populations can be associated with different accretion modes (cold-mode or hot-mode). We find that the cold-mode sources have a steeper spectral index and produce more luminous radio lobes, but generally reside in smaller host galaxies than their hot-mode counterparts. This can be attributed to the fact that they are accreting material more efficiently. Lastly, we compare the AT20G survey with the S-cubed semi-empirical (S3-SEX) models and conclude that the S3-SEX models need refining to correctly model the compact cores of AGN. The AT20G survey provides the ideal sample to do this. [ABSTRACT FROM AUTHOR]

We present radio and infrared observations of four hyper-compact H ii regions and four ultra-compact H ii regions in the southern Galactic plane. These objects were selected from a blind survey for ultra-compact H ii regions using data from two new radio surveys of the southern sky: the Australia Telescope 20 GHz survey and the second epoch Molonglo Galactic Plane Survey at 843 MHz. To our knowledge, this is the first blind radio survey for hyper- and ultra-compact H ii regions. We have followed up these sources with the Australia Telescope Compact Array to obtain H70α recombination line measurements, higher resolution images at 20 GHz and flux density measurements at 30, 40 and 95 GHz. From this, we have determined sizes and recombination line temperatures as well as modelling the spectral energy distributions to determine emission measures. We have classified the sources as hyper-compact or ultra-compact on the basis of their physical parameters, in comparison with benchmark parameters from the literature. Several of these bright, compact sources are potential calibrators for the low-frequency instrument (30–70 GHz) and the 100 GHz channel of the high-frequency instrument of the Planck satellite mission. They may also be useful as calibrators for the Australia Telescope Compact Array, which lacks good non-variable primary flux calibrators at higher frequencies and in the Galactic plane region. Our spectral energy distributions allow the flux densities within the Planck bands to be determined, although our high-frequency observations show that several sources have excess emission at 95 GHz (3 mm) that cannot be explained by current models. [ABSTRACT FROM AUTHOR]