A deep radio and optical survey near the North Galactic Pole - II. Magnitudes and colours of objects in the optical fields of 5C 12 radio sources

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Abstract
We describe an investigation of the objects near the positions of 299 radio sources found in the 5C 12 survey, based on 16 sky-limited, broadband plates taken with the 48-in. Schmidt telescope of Hale Observatories. Magnitudes and colours were derived for about 1200 objects in 1-arcmin2 fields at the radio positions. Magnitude calibration was carried out using a Racine wedge, with ΔM = 5.1 mag, and by taking plates of SA57 which overlapped plates of the 5C 12 area by 1.5°. The resulting measures of b, v and r are accurate to 0.2 mag at 22.4, 22.3 and 21.3 mag respectively. The results are compared with those from other investigations of faint objects at high galactic latitudes. In agreement with these, the majority of objects fainter than 20 mag are found to be galaxies, and the colour distribution of all stellar objects is bimodal in form. However, the colour distribution of stellar objects within 10 arcsec of the radio positions is not bimodal, and is representative of QSO colours. Galaxies close to the radio positions likewise exhibit a colour distribution which differs significantly from that of galaxies in the entire sample.

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