The cluster environments of powerful radio galaxies

Abstract

We have used the technique of angular cross-correlation to estimate the local galaxy density around powerful radio galaxies. Our results are based on the Lick galaxy counts for z < 0.1, with deep galaxy samples from UK Schmidt plates scanned with COSMOS used to extend the range to z < 0.25. We analyse the errors in the derived clustering strengths, including the effect of photometrically estimated redshifts. Results in the form of the ratio of the spatial cross-correlation amplitude (Bgg*) to the autocorrelation amplitude for galaxies in general (Bgg) are given for ∼200 radio sources, from which several important trends are evident:(i) As was well-known, the low-luminosity sources of Fanaroff–Riley class I lie on average in richer clusters than those of class II. There appears, however, to be a real scatter in properties: cluster strength is not the sole factor governing structure.(ii) We find no strong statistical evidence for the difference in environment which was previously suggested to exist between different sub-classes of the class II sources.(iii) Compact radio sources appear to lie in regions of low galaxy density, with (Bgg*/Bgg) a factor of ≳2 lower than for Fanaroff–Riley class I sources (and possibly even slightly lower than for Fanaroff–Riley class II sources). This result may pose a severe problem for unified relativistic beaming models.