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 \sim 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 _ { \nu } = 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 \sim 4 ) .