Measuring the Spin of Spiral Galaxies

We compute the angular momentum, the spin parameter, and the related distribution function for dark matter halos hosting a spiral galaxy. We rely on scaling laws, inferred from observations, that link the properties of the galaxy to those of the host halo; we further assume that the dark matter has the same total specific angular momentum of the baryons. Our main results are as follows: (1) We find that the gas component of the disk significantly contributes to the total angular momentum of the system. (2) By adopting for the dark matter the observationally supported Burkert profile, we compute the total angular momentum of the disk and its correlation with the rotation velocity. (3) We find that the distribution function of the spin parameter λ peaks at a value of about 0.03, consistent with a no-major-merger scenario for the late evolution of spiral galaxies.