Distant FR I radio galaxies in the Hubble Deep Field: implications for the cosmological evolution of radio-loud AGN

I A G Snellen, P N Best

Research output: Contribution to journalArticlepeer-review

Abstract

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 z > 1 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 z > 1 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.

Original languageEnglish
Pages (from-to)897-902
Number of pages6
JournalMonthly Notices of the Royal Astronomical Society
Volume328
Issue number3
DOIs
Publication statusPublished - 11 Dec 2001

Keywords

  • surveys
  • galaxies : active
  • cosmology : observations
  • radio continuum : galaxies
  • PHOTOMETRY
  • DICHOTOMY
  • EMISSION

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