Multi-frequency spectral analysis of extragalactic radio sources in the 33-GHz VSA catalogue: sources with flattening and upturn spectrum

TL;DR

This study analyzes the spectral properties of extragalactic radio sources at 33GHz, revealing that many exhibit spectral flattening or upturns at higher frequencies, which impacts future CMB observations.

Contribution

It provides the first detailed spectral analysis of VSA sources across 1.4 to 33GHz, identifying a significant fraction with upturn spectra and linking these to AGN core dominance.

Findings

Most sources show spectral flattening above 5GHz.

19% of sources have upturn spectra with low-frequency fall and high-frequency rise.

Spectral shapes support the AGN unified scheme and influence CMB observation strategies.

Abstract

We present an analysis of the spectral properties of the extragalactic radio sources in the nearly-complete VSA sample at 33GHz. Data from different surveys are used to study source spectra between 1.4 and 33GHz. We find that, in general, spectra can not be well described by a single power law in the range of frequencies considered. In particular, most of the VSA sources that are steep between 1.4 and 5GHz, show a spectral flattening at \nu>5GHz. We identify 20 objects (19% of the sample) clearly characterized by an upturn spectrum, i.e., a spectrum falling at low frequencies (\nu<5GHz) and inverted at higher frequencies. Spectra with high-frequency flattening or upturn shape are supposed to occur when the emission from the AGN compact core begins to dominate over the component from extended lobes. This picture fits well with the AGN unified scheme, for objects observed at intermediate viewing angles of the AGN jet. Finally, we discuss implications that this class of sources can have on future CMB observations at high resolution.