| 30th of November 2015 |
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| Planck-ATCA Co-eval Observations (PACO) project |
| by Marcella Massardi (INAF, Italy) |
| The Planck-ATCA Co-eval Observations (PACO)
project (Massardi
et al. 2015, MNRAS, in press)
has yielded observations of 464 sources with the Australia Telescope
Compact Array (ATCA) between 4.5 and 40 GHz. The main purpose of the project
was to investigate the spectral properties of mm-selected radio sources at
frequencies below and overlapping with the ESA's Planck satellite
frequency bands, minimizing the variability effects by observing almost
simultaneously with the first two Planck all-sky surveys. In this paper
we present the whole catalogue of observations in total intensity. By
comparing PACO with the various measures of Planck Catalog of Compact
Sources (PCCS) flux densities we found the best consistency with the
PCCS `detection pipeline' photometry (DETFLUX) that we used to
investigate the spectral properties of sources from 5 to 217 GHz. Of our
sources, 91 per cent have remarkably smooth spectrum, well described by
a double power-law over the full range. This suggests a single emitting
region, at variance with the notion that `flat' spectra result from the
superposition of the emissions from different compact regions,
self-absorbed up to different frequencies. Most of the objects show a
spectral steepening above 30 GHz, consistent with synchrotron emission
becoming optically thin. Thus, the classical dichotomy between
flat-spectrum/compact and steep-spectrum/extended radio sources, well
established at cm wavelengths, breaks down at mm wavelengths. The
mm-wave spectra do not show indications of the spectral break expected
as the effect of `electron ageing', suggesting young source ages. Figure caption: Averaged PACO spectra of sources with at least 2 PACO observations and 2 points in the Planck frequency bands (open circles) up to 217 GHz fitted with a double power law (dotted line). |