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Description
A deep multicolor optical identification program is presented for a complete sample of 302 radio sources that were observed with the Westerbork Synthesis Radio Telescope within 5.52{deg}^2^ down to S_1.4GHz_>~0.6mJy (5{sigma}). Optical identifications are made from multicolor prime focus plates taken with the Kitt Peak 4 meter telescope in the passbands U,J,F and N with approximate respective limiting magnitudes of ~23.3, 23.7, 22.7 and 21.1. The astrometry has systematic errors smaller than 0.2"-0.3", while the random errors are of order 0.4". The agreement between the radio and optical coordinate frames is on average better than 0.25". Identifications are based on positional coincidence using the likelihood ratio method. For 171 out of the 302 radio sources likely identifications are proposed with an a posteriori identification percentage of 53%, 14 expected spurious objects (or 5% of all radio sources), while not more than 4 identifications have been missed. The sample reliability is 92% and its completeness 98%. About 20% of the identifications are stellar objects, the remaining have extended images or are too faint to be classified. About 15% of the identifications appear in possible clusters. The identification statistics are roughly constant from field to field, except for the Hercules 2 field which has an unusually high identification fraction (74%). The identification fraction is presented as a function of 21 cm flux density and limiting magnitude, and amounts to 15% for 1<~S_1.4GHz_<~100mJy down to the Palomar Sky Survey limit and 29% down to the effective 48" Schmidt limit. For S_1.4_<~100mJy the identification fraction remains roughly constant with flux density, contrary to the prediction of higher fractions by some models for cosmological evolution. The magnitude distributions for galaxies plus objects of unknown type generally increase towards fainter magnitudes; the quasar magnitude distributions are also increasing but not as steeply as for the radio galaxies.
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