Deep 610-MHz Giant Metrewave Radio Telescope observations of the Spitzer extragalactic First Look Survey field - III. The radio properties of Infrared-Faint Radio Sources

TL;DR

This study investigates Infrared-Faint Radio Sources (IFRSs) using deep radio and infrared observations, revealing their properties and suggesting they are high-redshift, compact FRII radio galaxies based on spectral and size constraints.

Contribution

It provides the first detection and analysis of IFRSs in the Spitzer First Look Survey, characterizing their radio properties and proposing a high-redshift FRII galaxy model.

Findings

14 IFRSs detected in the survey field

Spectral indices range from 0.05 to 1.38

Likely high-redshift, compact FRII radio galaxies

Abstract

Infrared-Faint Radio Sources (IFRSs) are a class of source which are bright at radio frequencies, but do not appear in deep infrared images. We report the detection of 14 IFRSs within the Spitzer extragalactic First Look Survey field, eight of which are detected near to the limiting magnitude of a deep R-band image of the region, at R ~ 24.5. Sensitive Spitzer Space Telescope images are stacked in order to place upper limits on their mid-infrared flux densities, and using recent 610-MHz and 1.4-GHz observations we find that they have spectral indices which vary between alpha = 0.05 and 1.38, where we define alpha such that S = S_0 nu^(- alpha), and should not be thought of as a single source population. We place constraints on the luminosity and linear size of these sources, and through comparison with well-studied local objects in the 3CRR catalogue demonstrate that they can be modelled as being compact (< 20 kpc) Fanaroff-Riley Class II (FRII) radio galaxies located at high redshift (z > 4).