2008MNRAS.391.1000G

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 α = 0.05 and 1.38, where we define α such that S_ν = S₀ν^–α, 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 Revised Revised Third Cambridge catalogue demonstrate that they can be modelled as being compact (<20 kpc) Fanaroff-Riley type II (FRII) radio galaxies located at high redshift (z ∼ 4).