The structure of spiral galaxies: radial profiles in stellar mass-to-light ratio and the dark matter distribution

Context. The colour and metallicity gradients observed in spiral galaxies suggest that the mass-to-light ratio (M*/L) of the stellar disc is a function of radius. This is indeed predicted by chemo-photometric models of galactic discs. Aims. We investigate the distribution of luminous and dark matter in spiral galaxies, taking into account the radial dependence of the stellar M*/L, which is usually assumed to be constant in studies of the mass structure. Methods. From earlier chemo-photometric models and in agreement with the observed radial profiles of galaxy colours, we derive the typical average M*/L profile of the stellar discs of spiral galaxies. We computed the corresponding variable mass-to-light (VML) stellar surface density profile and then the VML disc contribution to the circular velocity. We used the latter, combined with a well-studied dark matter velocity profile, to mass model co-added rotation curves. Results. By investigating rotation curves in the framework of VML stellar discs, we confirm the scenario obtained with the constant M*/L assumption: a dark matter halo with a shallow core, an inner baryon-dominated region, and a larger proportion of dark matter in smaller objects. However, the resulting size of the the dark halo core and of the inner baryon dominance region are somewhat smaller. The stronger role that VML discs have in the innermost regions is important for constraining the galaxy mass structure in both Λ Cold Dark Matter and MOND scenarios.