THE UNIVERSAL ROTATION CURVE OF SPIRAL GALAXIES .1. THE DARK-MATTER CONNECTION

We use a homogeneous sample of about 1100 optical and radio rotation c urves (RCs); and relative surface photometry to investigate the main m ass structure properties of spirals, over a range of 6 mag and out to less than or similar to 1.5 and 2 optical radii (for the optical and r adio data, respectively). We confirm the strong dependence on luminosi ty for both the profile and the amplitude of RCs claimed by Persic & S alucci. Spiral RCs show the striking feature that a single global para meter, e.g. luminosity, dictates the rotational velocity at any radius for any object, so revealing the existence of a universal RC. At high luminosities, there is a slight discrepancy between the profiles of R Cs and those predicted from the luminous matter (LM) distributions: th is implies a small, yet detectable, amount of dark matter (DM). At low luminosities, the failure of the LM prediction is much more severe, a nd the DM is the only relevant mass component. We show that the univer sal RC implies a number of scaling properties between dark and luminou s galactic structure parameters: (i) the DM/LM mass ratio scales inver sely with luminosity; (ii) the central halo density scales as L(-0.7); (iii) the halo core radius is comparable to the optical radius, but s hrinks for low luminosities; (iv) the total halo mass scales as L(0.5) . Such scaling properties can be represented as a curve in the (lumino sity)-(DM/LM mass ratio)-(DM core radius)-(DM central density) space, which provides a geometrical description of the tight coupling between the dark and the luminous matter in spiral galaxies.