The Edinburgh/Durham Southern Galaxy Catalogue - II. Image classification and galaxy number counts

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Abstract
The Edinburgh/Durham Southern Galaxy Catalogue is the first large-scale machine based optical galaxy catalogue. This galaxy survey is based on digitized scans of the SERC(J) survey plates, using the COSMOS high-speed measuring machine. In this second paper of a series concerned with the catalogue we present our optimized procedure for minimizing both plate position dependent and magnitude dependent systematic errors in image classification as a star or galaxy. We demonstrate the classification techniques and strict quality control that are necessary in order to reduce misclassification to an acceptable level for the production of a homogeneous catalogue from photographic plates of varying quality. We apply a fully automated approach over the full magnitude range of the survey (bj < 20). This, coupled with our control of systematic errors, ensures that the Edinburgh/Durham Southern Galaxy Catalogue is deeper and more uniform than any existing optical survey. Statistics are presented to show the final catalogue is > 95 per cent complete with < 10 per cent stellar contamination and has only ≃ 3 per cent rms residual variation in the number of objects classed as galaxies. Number–magnitude counts are presented for galaxies in a 100 deg2 region of our survey down to bj ∼ 19.5. These are used to place constraints on the level of stellar contamination in our survey. In addition, since the 100 deg2 area is considerably larger than any previous number count survey region, these new data provide the most reliable estimate of galaxy counts at bright magnitudes. There is significant deviation between our counts and a no-evolution model for bj > 18.75.

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