The Planck-ATCA Co-eval Observations (PACO) project: analysis of radio source properties between 5 and 217 GHz

TL;DR

This study presents a comprehensive analysis of radio source spectral properties between 5 and 217 GHz, revealing that most sources have smooth spectra and challenging traditional classifications at mm wavelengths.

Contribution

The paper provides the first extensive catalog of co-eval radio source observations from 5 to 217 GHz, demonstrating spectral smoothness and the breakdown of classical dichotomies at mm wavelengths.

Findings

91% of sources have smooth, double power law spectra

Most sources show spectral steepening above 30 GHz

Spectral data suggest young source ages, with no spectral break from electron ageing

Abstract

The Planck-ATCA Co-eval Observations (PACO) project 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% 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.