A deep radio/optical survey near the North Galactic Pole - III. A 4.85-GHz survey of the 5C12 area

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Abstract
The 100-m Effelsberg telescope of MPIfR has been used to carry out a 4.85-GHz survey of the 5C12 area. The survey is complete to 18 mJy, and 93 sources are catalogued. Positions and flux densities are compared with other available data to check both random and systematic errors, and in particular to test for radial polar-diagram errors in the 5C12 survey. Source counts from the catalogue agree with those from other surveys, confirming the drop below a Euclidean law towards mJy levels, and providing no evidence of anisotropy on scales of degrees. The spectral-index distribution shows that a dramatic change in the source population takes place as the 5-GHz flux density level is decreased: the distribution is dominated by steep-spectrum sources, unlike the bimodal (flat-spectrum and steep-spectrum) distribution seen at Jy-levels. Comparison of long- and short-baseline spectral-index distributions shows that this change cannot be due to the presence of high-redshift objects with spectral curvature.

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