1997A&A...318..741S

The usual bulge/disk decomposition of the surface brightness distribution in disk galaxies generally gives good results for S0 galaxies, but often fails for more advanced types of the Hubble sequence. We propose in this paper that, in the case of spiral galaxies, a third component must be considered: the contribution of spiral arms. The functional form of the luminosity law of spiral arms is here obtained by means of the density wave theory, in contrast with the essentially empirical nature of the bulge and disk laws. An advantage of this approach is that the fitting parameters appearing in our analytical expression are related to galactic dynamical properties as the mass-to-luminosity ratio, the angular velocity of the spiral pattern rotation, etc. Consequently, the fitting of surface brightness profiles provides an estimate of such quantities. Reciprocally, the requirement that galactic physical properties must be reasonable can be used as a condition limiting the freedom in the fitting of data. Our luminosity law has been applied to some spiral galaxies for which the simple bulge/disk decomposition did not give good results. We find that our expression provides excellent fits to the observed profiles of these galaxies, and implies physical properties which reasonably coincide with those obtained from other procedures.