THE T-SYSTEM - A NEW SYSTEM FOR ESTIMATING THE TOTAL MAGNITUDES OF GALAXIES

We present a new, but simple, procedure for estimating the total magni tudes of galaxies. This procedure involves the out-focusing of digital galaxy images numerically! the fitting of the resulting surface-brigh tness profiles with a single generalised profile model and the extrapo lation of the fitted profiles to infinite radial distances. This new s ystem, which we denote t, differs fundamentally from the T system (of the Reference Catalog of Bright Galaxies series in that: (1) it enable s a galaxy's luminosity profile to be extrapolated without the need fo r any prior morphological classification, and (2) it is applicable to images of widely different spatial resolutions (including unresolved o nes) because it takes into account systematic effects due to different ial image resolution. It also differs fundamentally from the Kron syst em in that: (1) it can be derived directly from surface photometry wit hout the need to go back to the plate scans or CCD frames (unless the surface photometry is of high resolution and/or the galaxies being mea sured are very bright), and (2) it can cope with merged images (provid ed they are separable by image-segmentation software). Through worked examples, we demonstrate the stability of t-system total magnitudes wi th respect to morphological type, the seeing conditions at the time of observation, degree of smoothing and limiting isophote. We also compa re and contrast the new system with both the T system and the Kron sys tem, and investigate the advantages and limitations of each of the thr ee systems.