Angular diameter counts of galaxies: a method for determining the selection criteria for deep Schmidt plates

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Abstract
It is demonstrated that a sensitive method for studying the selection effects that operate on galaxy samples is to investigate theoretical fits to the observed angular diameter frequency distribution. This technique is applied to a two-colour sample taken from SRC 48-in Schmidt plates that have been measured by the COSMOS machine. The selection effects are discussed in terms of the selection function, i.e. the probability that an arbitrary galaxy at a certain redshift is accepted for the sample. The results show that sky-limited R plates reach considerably deeper than sky-limited J plates and that it is ‘seeing’ that is the most important process accounting for the loss of faint galaxies. Furthermore, it is found necessary to invoke luminosity evolution in order to account for the slope of the observed distribution at large angular diameters.

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