2001MNRAS.328..897S

Deep and high-resolution radio observations of the Hubble Deep Field and flanking fields have shown the presence of two distant edge-darkened FRI radio galaxies, allowing for the first time an estimate of their high-redshift space density. If it is assumed that the space density of FRI radio galaxies at z>1 is similar to that found in the local Universe, then the chance of finding two FRI radio galaxies at these high radio powers in such a small area of sky is <1 per cent. This suggests that these objects were significantly more abundant at z>1 than at present, effectively ruling out the possibility that FRI radio sources undergo no cosmological evolution. We suggest that FRI and FRII radio galaxies should not be treated as intrinsically distinct classes of objects, but that the cosmological evolution is simply a function of radio power with FRI and FRII radio galaxies of similar radio powers undergoing similar cosmological evolutions. Since low-power radio galaxies have mainly FRI morphologies and high-power radio galaxies have mainly FRII morphologies, this results in a generally stronger cosmological evolution for the FRIIs than the FRIs. We believe that additional support from the V/V_max test for evolving and non-evolving population of FRIIs and FRIs respectively is irrelevant, since this test is sensitive over very different redshift ranges for the two classes.