Open AccessDistant FR I radio galaxies in the Hubble Deep Field: implications for the cosmological evolution of radio‐loud AGN
Author(s) -
Snellen I.A.G.,
Best P.N.
Publication year - 2001
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2001.04947.x
Subject(s) - physics , astrophysics , radio galaxy , redshift , galaxy , astronomy , hubble deep field , x shaped radio galaxy , population , galaxy formation and evolution , demography , sociology
Deep and high‐resolution radio observations of the Hubble Deep Field and flanking fields have shown the presence of two distant edge‐darkened FR I radio galaxies, allowing for the first time an estimate of their high‐redshift space density. If it is assumed that the space density of FR I radio galaxies at is similar to that found in the local Universe, then the chance of finding two FR I 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 than at present, effectively ruling out the possibility that FR I radio sources undergo no cosmological evolution. We suggest that FR I and FR II 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 FR I and FR II radio galaxies of similar radio powers undergoing similar cosmological evolutions. Since low‐power radio galaxies have mainly FR I morphologies and high‐power radio galaxies have mainly FR II morphologies, this results in a generally stronger cosmological evolution for the FR IIs than the FR Is. We believe that additional support from the V / V max test for evolving and non‐evolving population of FR IIs and FR Is respectively is irrelevant, since this test is sensitive over very different redshift ranges for the two classes.