The global structure of galactic discs

A statistical study of global galaxy parameters can help to improve our understanding of galaxy formation processes. In this paper we present the analysis of global galaxy parameters based on optical and near-infrared observations of a large sample of edge-on disc galaxies. We found a correlation between the ratio of the radial to vertical scale parameter and galaxy type: galaxies become systematically thinner when going from type SO to type Sc, whereas the distribution seems to level off for later types. The observed scalelength ratios (and thus the radial colour gradients) largely represent the dust content of the galaxies. On average, the colour gradients indicated by the scalelength ratios increase from type Sa to at least type Sc. For galaxy types later than Sc, the average colour gradient seems to decrease again. The distribution of K-band (edge-on) disc central surface brightnesses is rather flat, although with a large scatter. However, the latest-type sample galaxies (T > 6) show an indication that their average disc central surface brightnesses may be fainter than those of the earlier types. This effect is probably not the result of dust extinction.