THE FAINT END OF THE ZWICKY MAGNITUDE SCALE - CCD ISOPHOTAL MAGNITUDES VERSUS ZWICKY MAGNITUDES

We investigate the isophotal nature of the Zwicky magnitude scale at the faint end (mZwicky≥14.5) by comparing Zwicky magnitudes to blue isophotal magnitudes which have been derived from surface photometry of digital galaxy images. Our procedure is to step out from the center of the galaxy in surface brightness until we reach the isophote whose enclosed light yields the cataloged Zwicky magnitude. In so doing we find that, in the mean, the Zwicky magnitude closely corresponds to the blue magnitude measured within the B = 26.0 mag arcsec-2 isophote. However, the scatter is 0.31 mag and thus Zwicky magnitudes, when used as a luminosity indicator, are only accurate to ≈30%. Moreover, the distribution of isophotal surface brightness is very broad and there are a substantial number of galaxies where the Zwicky magnitude corresponds to B = 22.5 to 24.0 magarcsec-2. This behavior suggests that the Zwicky magnitude scale is neither well-defined nor sky limited. We also find evidence that suggests that the Zwicky magnitude scale behaves differently for spirals than for ellipticals. Finally, we show that even relative differences in Zwicky magnitude do not provide an accurate gauge of the true relative luminosities of galaxies. Altogether, these results strongly argue for the initiation of a Digital Sky Survey for the purposes of doing redshift surveys down to a well-known magnitude completeness limit.