Disk-bulge decompositions of spiral galaxies in UBVRI

A sample of 26 bright spiral galaxies (B-tot < 12.7) with low to medium inclination and without a bar was observed with UBVRI filters. The CAFOS focal reducer camera at the Calar Alto 2.2 m telescope was used. The surface-brightness distributions were fitted using a 2-dimensional algorithm with corresponding functions for the disk- and bulge-structure. For the disks an exponential function was used, for the bulges a Sersic R-beta law, was applied with the concentration parameter beta = 1/n as another fit variable. Correlations of the resulting structural parameters of disks and bulges in UBVRI are investigated, giving clues to the formation history of the bulges. We confirm that the large and bright bulges of early-type spirals are similar to elliptical galaxies. They were probably formed prior to the disks in a monolithic collapse or via early mergers. Late-type spirals have tiny and faint bulges with disk- like profiles. These bulges were probably formed after the disk in secular evolution processes, e. g. from a disk instability. The comparison of the color indices of corresponding spirals and bulges with population synthesis computations support above formation scenarios.