Ultra Steep Spectrum ( USS ) radio sources have been successfully used to select powerful radio sources at high redshifts ( z \gtrsim 2 ) .
Typically restricted to large-sky surveys and relatively bright radio flux densities , it has gradually become possible to extend the USS search to sub-mJy levels , thanks to the recent appearance of sensitive low-frequency radio facilities .
Here a first detailed analysis of the nature of the faintest USS sources is presented .
By using Giant Metrewave Radio Telescope and Very Large Array radio observations of the Lockman Hole at 610 MHz and 1.4 GHz , a sample of 58 USS sources , with 610 MHz integrated fluxes above 100 \mu Jy , is assembled .
Deep infrared data at 3.6 and 4.5 \mu m from the Spitzer Extragalactic Representative Volume Survey ( SERVS ) is used to reliably identify counterparts for 48 ( 83 % ) of these sources , showing an average total magnitude of [ 3.6 ] _ { AB } = 19.8 mag .
Spectroscopic redshifts for 14 USS sources , together with photometric redshift estimates , improved by the use of the deep SERVS data , for a further 19 objects , show redshifts ranging from z = 0.1 to z = 2.8 , peaking at z \sim 0.6 and tailing off at high redshifts .
The remaining 25 USS sources , with no redshift estimate , include the faintest [ 3.6 ] magnitudes , with 10 sources undetected at 3.6 and 4.5 \mu m ( typically [ 3.6 ] \gtrsim 22 - 23 mag , from local measurements ) , which suggests the likely existence of higher redshifts among the sub-mJy USS population .
The comparison with the Square Kilometre Array Design Studies Simulated Skies models indicate that Fanaroff-Riley type I radio sources and radio-quiet Active Galactic Nuclei may constitute the bulk of the faintest USS population , and raises the possibility that the high efficiency of the USS technique for the selection of high redshift sources remains even at the sub-mJy level .