The Deep SWIRE Field. IV. First properties of the sub-mJy galaxy population: redshift distribution, AGN activity and star formation

TL;DR

This study analyzes the properties and evolution of sub-mJy radio galaxies in the Deep SWIRE Field, revealing diverse galaxy types and suggesting many are transitional objects with complex AGN and star formation activity.

Contribution

It provides detailed multi-wavelength analysis of faint radio sources, estimating redshifts, stellar masses, and classifying galaxy types, highlighting the complexity of faint radio populations.

Findings

Significant population of green-valley galaxies observed.

Faint radio sources often are composite or transition objects.

The relative contribution of AGN and star-forming galaxies varies with flux density.

Abstract

We present a study of a 20cm selected sample in the Deep SWIRE VLA Field, reaching a limiting flux density of ~13.5 uJy at the image center. In a 0.6x0.6 square degrees field, we are able to assign an optical/IR counterpart to 97% of the radio sources. Up to 11 passbands from the NUV to 4.5um are then used to sample the spectral energy distribution (SED) of these counterparts in order to investigate the nature of the host galaxies. By means of an SED template library and stellar population synthesis models we estimate photometric redshifts, stellar masses, and stellar population properties, dividing the sample in three sub-classes of quiescent, intermediate and star-forming galaxies. We focus on the radio sample in the redshift range 0.3<z<1.3 where we estimate to have a redshift completeness higher than 90%, and study the properties and redshift evolution of these sub-populations. We find that, as expected, the relative contributions of AGN and star-forming galaxies to the uJy population depend on the flux density limit of the sample. At all flux levels a significant population of "green-valley" galaxies is observed. While the actual nature of these sources is not definitely understood, the results of this work may suggest that a significant fraction of faint radio sources might be composite (and possibly transition) objects, thus a simple "AGN vs star-forming" classification might not be appropriate to fully understand what faint radio populations really are.