A census of radio-selected AGN on the COSMOS field and of their FIR properties

TL;DR

This study provides a comprehensive census of radio-selected AGN in the COSMOS field, analyzing their FIR properties, multi-wavelength emission, and evolutionary stages, revealing correlations between luminosity, redshift, and host galaxy characteristics.

Contribution

It offers the first complete, statistically robust classification of radio AGN and their FIR emission properties across a wide redshift range, highlighting evolutionary trends.

Findings

Powerful radio AGN are more likely to emit in FIR at earlier epochs.

FIR emission in AGN is linked to star formation within hosts.

Different multi-wavelength emissions suggest an evolutionary sequence for AGN.

Abstract

We use the new catalogue by Laigle et al. (2016) to provide a full census of VLA-COSMOS radio sources. We identify 90% of such sources and sub-divide them into AGN and star-forming galaxies on the basis of their radio luminosity. The AGN sample is COMPLETE with respect to radio selection at all z<3.5. Out of 704 AGN, 272 have a counterpart in the Herschel maps. By exploiting the better statistics of the new sample, we confirm the results of Magliocchetti et al. (2014): the probability for a radio-selected AGN to be detected at FIR wavelengths is both a function of radio luminosity and redshift, whereby powerful sources are more likely FIR emitters at earlier epochs. Such an emission is due to star-forming processes within the host galaxy. FIR emitters and non-FIR emitters only differentiate in the z<1 universe. At higher redshifts they are indistinguishable from each other, as there is no difference between FIR-emitting AGN and star-forming galaxies. Lastly, we focus on radio AGN which show AGN emission at other wavelengths. We find that MIR emission is mainly associated with ongoing star-formation and with sources which are smaller, younger and more radio luminous than the average parent population. X-ray emitters instead preferentially appear in more massive and older galaxies. We can therefore envisage an evolutionary track whereby the first phase of a radio-active AGN and of its host galaxy is associated with MIR emission, while at later stages the source becomes only active at radio wavelengths and possibly also in the X-ray.