The Planck-ATCA Co-eval Observations project: the spectrally selected sample

Abstract

The Planck Australia Telescope Compact Array (Planck-ATCA) Co-eval Observations (PACO) have provided multi-frequency (5-40 GHz) flux density measurements of complete samples of Australia Telescope 20 GHz (AT20G) radio sources at frequencies below and overlapping with Planck frequency bands, almost simultaneously with Planck observations. In this work we analyse the data in total intensity for the spectrally selected PACO sample, a complete sample of 69 sources brighter than S_{20 GHz} = 200 mJy selected from the AT20G survey catalogue to be inverted or upturning between 5 and 20 GHz. We study the spectral behaviour and variability of the sample. We use the variability between AT20G (2004-2007) and PACO (2009-2010) epochs to discriminate between candidate High-Frequency Peakers (HFPs) and candidate blazars. The HFPs picked up by our selection criteria have spectral peaks >10 GHz in the observer frame and turn out to be rare (<0.5 per cent of the S_{20 GHz} ≥ 200 mJy sources), consistent with the short duration of this phase implied by the `youth' scenario. Most (≃ 89 per cent) of blazar candidates have remarkably smooth spectra, well described by a double power law, suggesting that the emission in the PACO frequency range is dominated by a single emitting region. Sources with peaked PACO spectra show a decrease of the peak frequency with time at a mean rate of -3 ± 2 GHz yr^-1 on an average time-scale of <τ> = 2.1 ± 0.5 yr (median: τ_median = 1.3 yr). The 5-20 GHz spectral indices show a systematic decrease from AT20G to PACO. At higher frequencies spectral indices steepen: the median α⁴⁰₃₀ is steeper than the median α²⁰₅ by δα = 0.6. Taking further into account the Wide-field Infrared Survey Explorer data we find that the Spectral Energy Distributions (SEDs), νS(ν), of most of our blazars peak at ν^SED_p < 10⁵ GHz; the median peak wavelength is λ^SED_p ≃ 93 μm. Only six have ν^SED_p > 10⁵ GHz.