The spatial clustering of radio sources in NVSS and FIRST; implications for galaxy clustering evolution

We have measured the angular correlation function, w(theta ), of radio sources in the 1.4 GHz NVSS and FIRST radio surveys. Below ~ 6arcmin the signal is dominated by the size distribution of classical double radio galaxies, an effect underestimated in some previous studies. We model the physical size distribution of FRII radio galaxies to account for this excess signal in w(theta ). The amplitude of the true cosmological clustering of radio sources is roughly constant at A =~ 1*E^-3 for flux limits of 3-40 mJy, but has increased to A =~ 7*E^-3 at 200 mJy. This can be explained if powerful (FRII) radio galaxies probe significantly more massive structures compared to radio galaxies of average power at z ~ 1. This is consistent with powerful high-redshift radio galaxies generally having massive (forming) elliptical hosts in rich (proto-)cluster environments. For FRIIs we derive a spatial (comoving) correlation length of r₀=14+/-3 h^-1 Mpc. This is remarkably close to that measured for extremely red objects (EROs) associated with a population of old elliptical galaxies at z ~ 1 by \citet{daddi01}. Based on their similar clustering properties, we propose that EROs and powerful radio galaxies may be the same systems seen at different evolutionary stages. Their r₀ is ~ 2x higher than that of QSOs at a similar redshift, and comparable to that of bright ellipticals locally. This suggests that r₀ (comoving) of these galaxies has changed little from z ~ 1 to z=0, in agreement with current Lambda CDM hierarchical merging models for the clustering evolution of massive early-type galaxies. Alternatively, the clustering of radio galaxies can be explained by the galaxy conservation model. This then implies that radio galaxies of average power are the progenitors of the local field population of early-types, while the most powerful radio galaxies will evolve into a present-day population with r₀ comparable to that of local rich clusters.