Total Magnitudes of Virgo Galaxies. II. An Investigation into the mp Scale of Volume I of Zwicky et al.'s Catalogue of Galaxies and of Clusters of Galaxies

We investigate the photographic-magnitude (m^p) scale of Volume I of the Catalogue of Galaxies and of Clusters of Galaxies for galaxies in the Virgo direction. It is found that for 11.5 <= Bt < 14 5, the m^p values listed correspond very closely (with a scatter of only 0.16 magnitude) to blue magnitudes measured to the μ_B = 24 4 isophote. If m^p values need to be used as estimates of total blue magnitude the scatter is 0.27 magnitude over the same range in B_t and the mean offset is m^p-B^t=0 28 magnitude. However, a serious scale error at the bright end causes both the isophotal and total luminosities of galaxies brighter than B^t~11.5 to be severely underestimated. At the faint end there also appears to be a significant scale error. This causes galaxy luminosities to be seriously overestimated faintward of B^t ~ 14.5. We demonstrate that this is a real effect based on a detailed galaxy-by-galaxy study of the catalogue's completeness down to B^t= 17.0. The catalogue is found only to be complete to B^t ~ 14.7 whilst its degree of incompleteness is found to be relatively constant at the 30% level over the range 14.75 <= B^t < 15.75. Most of the missing objects are found to be elliptical galaxies and so late-type objects are over-represented at the faint end.