Optical identifications of ∼ 4000 radio sources at the 1 mJy level

We present an analysis of the properties of optical counterparts of radio sources down to 1 mJy. Optical identifications have been obtained by matching together objects from the APM and FIRST surveys over the region 148.13◦ ∼ < RA(2000) ∼ < 218.13◦, −2.77◦ ∼ Dec ∼ 2.25 ◦. Selecting radio sources down to 1 mJy, and adopting a uniform optical limit of bJ = 21.5 we find 3176 have a counterpart in the APM catalogue, corresponding to 13 per cent of the radio sample. For bJ ≤ 20.5 we can divide radio sources into resolved radio galaxies and stellarlike objects (principally QSO). We find the population of radio galaxies to be mainly made by early-type galaxies with very red colours (bJ −R up to ∼ 4) and a radio-tooptical ratio 10 ∼ r ∼ 10 . The contribution of starbursting objects is negligible. In general QSO show r ∼ > 10 and 0 ∼ < bJ −R ∼ < 1. On the basis of the R magnitudes, we estimate the sample of radio galaxies to be complete up to z ≃ 0.3. We can therefore divide the whole sample of radio sources into a low-z and high-z population. The lowz one includes the objects identified as galaxies in the APM survey, the high-z one includes sources either identified as QSO or with no optical counterpart for bJ ≤ 20.5. We find that radio galaxies are strongly clustered and highly biased tracers of the underlying mass distribution. Models for the angular correlation function w(θ) show good agreement with the observations if we assume a bias factor b ≃ 2 at z ≃ 0.3.