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

Abstract

We have measured the angular correlation function of radio sources in the 1.4 GHz NVSS and FIRST surveys. Below ~6 arcminutes w(theta) is dominated by the size distribution of radio galaxies. A model of the size distribution of radio galaxies can account for this excess signal in w(theta). The amplitude of the cosmological clustering of radio sources is roughly constant at A~0.001 from 3 to 40 mJy, but has increased to A~0.007 at 200 mJy. This can be explained if powerful (FRII) radio galaxies probe more massive structures at z~1 compared to average power radio galaxies, consistent with powerful high-z radio galaxies generally having massive (forming) elliptical hosts in rich cluster environments. For FRIIs we derive a spatial (comoving) correlation length of r_0=14\pm3 h^{-1} Mpc. This is close to that measured for extremely red objects (EROs) associated with a population of old elliptical galaxies at z~1 by Daddi et al. (2001). 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 r_0 is ~2 times higher than that of QSOs at a similar redshift, and comparable to that of bright ellipticals locally. This suggests that r_0 (comoving) of these galaxies has changed little from z~1 to z=0, in agreement with current LCDM hierarchical models for 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 early-type field population, while the most powerful radio galaxies will evolve into a present-day population with r_0 similar to that of local rich clusters.