Polarisation of mJy radio sources

TL;DR

This paper investigates the polarisation properties of faint radio sources around 1 mJy, revealing that unresolved spiral galaxies can be highly polarised and providing predictions for polarised source counts relevant for SKA observations.

Contribution

It demonstrates that unresolved spiral galaxies can have high polarisation fractions and offers new predictions for polarised source counts at microjansky levels, informing SKA surveys.

Findings

Unresolved spiral galaxies can be polarised up to ~20% at 4.8 GHz.

Polarised source counts vary among AGN and star-forming galaxies.

Predictions extend to microjansky flux levels for future radio surveys.

Abstract

Predictions of the number of faint polarised radio sources that can be detected by SKA pathfinder telescopes and the SKA depend on the polarisation properties of radio sources with a total flux density around 1 mJy. Total intensity source counts suggest a transition in the dominant population from AGN to galaxies around this flux density, and the properties of brighter radio sources may not be representative for this fainter population. We show that unresolved spiral galaxies can be highly polarised radio sources, up to ~ 20% polarised at 4.8 GHz. This result is partly based on observations of nearby galaxies, including galaxies with significant deviations from axial symmetry and other peculiarities. A first analysis of polarised source counts divided into steep-spectrum AGN, flat-spectrum AGN and star forming galaxies is presented, including a prediction of polarised source counts to microjansky levels.