The cluster environments of the z similar to 1 3CR radio galaxies

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An analysis of the environments around a sample of 28 3CR radio galaxies with redshifts 0.6 < z < 1.8 is presented, based primarily upon K-band images down to K similar to 20 taken using the UK Infrared Telescope (UKIRT). A net overdensity of K-band galaxies is found in the fields of the radio galaxies, with the mean excess counts being comparable to that expected for clusters of Abell Class 0 richness. A sharp peak is found in the angular crosscorrelation amplitude centred on the radio galaxies that, for reasonable assumptions about the luminosity function of the galaxies, corresponds to a spatial cross-correlation amplitude between those determined for low-redshift Abell Class 0 and 1 clusters.

These data are complemented by J-band images also from UKIRT, and by optical images from the Hubble Space Telescope. The fields of the lower redshift (z less than or similar to 0.9) radio galaxies in the sample generally show well-defined near-infrared colour-magnitude relations with little scatter, indicating a significant number of galaxies at the redshift of the radio galaxy; the relations involving colours at shorter wavelengths than the 4000 Angstrom bleak show considerably greater scatter, suggesting that many of the cluster galaxies have low levels of recent or ongoing star formation. At higher redshifts the colour-magnitude sequences are less prominent owing to the increased field galaxy contribution at faint magnitudes, but there is a statistical excess of galaxies with the very rod infrared colours (J - K greater than or similar to 1.75) expected of old cluster galaxies at these redshifts.

Although these results are appropriate for the mean of all of the radio galaxy fields, there exist large field-to-field variations in the richness of the environments. Many, but certainly not all, powerful z similar to 1 radio galaxies lie in (proto)cluster environments.

Original languageEnglish
Pages (from-to)720-736
Number of pages17
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 21 Sep 2000


  • galaxies : active
  • galaxies : clusters : general
  • galaxies : luminosity function, mass function
  • radio continuum : galaxies


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