The distribution of luminosity in spiral galaxies

Luminosity profiles derived from photographic surface photometry measurements for 26 spiral galaxies are presented and analyzed with regard to galactic structural parameters. The luminosity profiles, which were obtained for 2592 photometric measurements for each of the galaxies, are represented as elliptically averaged profiles. A decomposition procedure based on a de Vaucouleurs law for the bulge and a radial exponential law for the bulge is applied to obtain photometric parameters for the disk and bulge separately, as well as accurate inclinations and true bulge flattenings for some of the objects. Comparison of the actual profiles with the decomposition models reveals that departures from exponential disks generally occur when recent star formation is limited to certain disk regions, or when some perturbing feature such as a bar, companion or oval distortion is present. The distributions of disk central surface brightness, disk scale length, and true bulge flattening are compared with previous investigations, and it is found that the bulge-to-disk ratio is related to Hubble type, although with a scatter larger than observational uncertainties. Finally, a comparison of the bulge-to-disk mass ratios for the present spirals with those measured for S0s suggests the presence of intrinsic differences between the disks of the two types.