Several recent studies indicate that bulges are more complex than merely structureless relaxed stellar systems. We study the HST images of a sample of 130 nearby early type (S0-Sab) disc galaxies and detect pure structureless bulges with the Sersic i
ndex n > 2 for only 12% of the galaxies. Other galaxies show varied substructure in their inner regions (inner bars, inner spiral arms, inner rings) and sometimes contain no bulge at all. Inner substructure is more common for these galaxies, which also display structure at larger scales.
We construct a structural model of the Andromeda Galaxy, simultaneously corresponding to observed photometrical and kinematical data and chemical abundances. In this paper we present the observed surface brightness, colour and metallicity distributio
ns, and compare them to the model galaxy. In Paper II (Tempel, Tamm & Tenjes 2007) we present similar data for the kinematics, and derive the mass distribution of the galaxy. On the basis of U, B, V, R, I and L luminosity distributions, we construct the model galaxy as a superposition of four axially symmetric stellar components: a bulge, a disc, an inner halo and an extended diffuse halo. By using far-infrared imaging data of M31 and a thin dust disc assumption, we derive dust-free surface brightness and colour distributions. We find the total absorption corrected luminosity of M31 to be L_B = (3.3+/-0.7)x10^10 L_sun, corresponding to an absolute luminosity M_B = -20.8+/-0.2 mag. Of the total luminosity, 41% (0.57 mag) is obscured from us by the dust inside M31. Using chemical evolution models, we calculate mass-to-light ratios of the components, correspoding to the colour indices and metallicities. We find the total intrinsic mass-to-light ratio of the visible matter to be M/L_B=3.1-5.8 M_sun/L_sun and the total mass of visible matter M_vis =(10-19)x10^10 M_sun. The use of the model parameters for a dynamical analysis and for determining dark matter distribution is presented in Paper II.
