2000ApJ...531..701G

The mutual dynamical evolution of visible and dark matter in spiral galaxies may have resulted in some kind of coupling between the distributions of visible and dark matter today. This conjecture is empirically explored in the present paper, where rotation curves of 60 spiral galaxies and universal rotation curves are fitted using dark halo models with a distribution that depends on the luminous mass distribution. It is shown that the dark matter profiles of any universal rotation curve can be decomposed into two components: (1) a main component, called ``coupled halo'', and (2) a component having a gaslike distribution, negligible in bright galaxies, but of increasing significance toward faint galaxies. Once the disk component (stars, gas, and gaslike component) is subtracted, the dark halo integrated surface densities, σ_d(r), are in a plane (in log scale) σ²_d(r)=σ_γσ(r), where the fundamental parameters are the disk integrated surface density, σ(r), and a surface density, σ_γ, which depends on the galaxy system only and characterizes the relative importance of the dark halo to the disk mass. This is the coupled halo. In the case of an exponential disk, the coupled halo has a central profile of the form ρ∝r^–1 and a flat curve at r≥r_opt. The fact that the gaslike component decreases with luminosity suggests that it may be transformed into stars, and therefore could be dark baryons, possibly cold gas in the disk. In these models, the baryonic fraction (stars, gas, and gaslike component) is almost a constant over a range of 5 mag, that is, ∼30% at 1.5 optical radii. The stellar fraction of baryonic matter increases with luminosity. The model predicts a large fraction of gaslike baryonic dark matter in faint spiral galaxies, i.e., in H I gas-rich systems. The mass fraction of this gaslike component is negligible in a galaxy like the Milky Way, and reaches half the halo mass in the faint, low surface brightness galaxy DDO 154. Some fine-tuning relations, which look mysterious in the context of isothermal halos, are deduced from the coupled halos. The Tully-Fisher relation results from the coupling relation between visible and dark matter surface densities and from the virial theorem. The empirical coupling relation can be used in simulations of the dynamical evolution of gas and stars plus dark matter subsystems to constrain the nature of the halo dark matter. If a form of halo dark matter complies with the coupling relation, it will fit rotation curves without fine-tuning conspiracies.