The local radio-galaxy population at 20 GHz

We have made the first detailed study of the high-frequency radio-source population in the local universe, using a sample of 202 radio sources from the Australia Telescope 20 GHz (AT20G) survey identified with galaxies from the 6dF Galaxy Survey (6dFGS). The AT20G-6dFGS galaxies have a median redshift of z=0.058 and span a wide range in radio luminosity, allowing us to make the first measurement of the local radio luminosity function at 20 GHz. Our sample includes some classical FR-1 and FR-2 radio galaxies, but most of the AT20G-6dFGS galaxies host compact (FR-0) radio AGN which appear lack extended radio emission even at lower frequencies. Most of these FR-0 sources show no evidence for relativistic beaming, and the FR-0 class appears to be a mixed population which includes young Compact Steep-Spectrum (CSS) and Gigahertz-Peaked Spectrum (GPS) radio galaxies. We see a strong dichotomy in the Wide-field Infrared Survey Explorer (WISE) mid-infrared colours of the host galaxies of FR-1 and FR-2 radio sources, with the FR-1 systems found almost exclusively in WISE `early-type galaxies and the FR-2 radio sources in WISE `late-type galaxies. The host galaxies of the flat- and steep-spectrum radio sources have a similar distribution in both K--band luminosity and WISE colours, though galaxies with flat-spectrum sources are more likely to show weak emission lines in their optical spectra. We conclude that these flat-spectrum and steep-spectrum radio sources mainly represent different stages in radio-galaxy evolution, rather than beamed and unbeamed radio-source populations.

The extragalactic radio-source population at 95 GHz

We have used the Australia Telescope Compact Array (ATCA) at 95GHz to carry out continuum observations of 130 extragalactic radio sources selected from the Australia Telescope 20GHz (AT20G) survey. Over 90% of these sources are detected at 95 GHz, and we use a triple-correlation method to measure simultaneous 20 and 95 GHz flux densities. We show that the ATCA can measure 95GHz flux densities to ~10% accuracy in a few minutes for sources above ~50mJy. The median 20-95GHz spectral index does not vary significantly with flux density for extragalactic sources with S20>150 mJy. This allows us to estimate the extragalactic radio source counts at 95GHz by combining our observed 20-95GHz spectral-index distribution with the accurate 20GHz source counts measured in the AT20G survey. The resulting 95GHz source counts down to 80 mJy are significantly lower than those found by several previous studies. The main reason is that most radio sources with flat or rising spectra in the frequency range 5-20GHz show a spectral turnover between 20 and 95 GHz. As a result, there are fewer 95GHz sources (by almost a factor of two at 0.1 Jy) than would be predicted on the basis of extrapolation from the source populations seen in lower-frequency surveys. We also derive the predicted confusion noise in CMB surveys at 95GHz and find a value 20-30% lower than previous estimates. The 95GHz source population at the flux levels probed by this study is dominated by QSOs with a median redshift z~1. We find a correlation between optical magnitude and 95GHz flux density which suggests that many of the brightest 95 GHz sources are relativistically beamed, with both the optical and millimetre continuum significantly brightened by Doppler boosting.