Empirical distributions of galactic $λ$ spin parameters from the SDSS

Abstract

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulas to derive an estimate of the halo spin parameter $λ$ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of $λ$, which we apply to a large volume limited sample of galaxies taken from the SDSS data base. The large numbers involved (11,597) allow the derivation of reliable $λ$ distributions, as signal adds up significantly in spite of the errors in the inferences for particular galaxies. We find that if the observed distribution of $λ$ is modeled with a log-normal function, as often done for this distribution in dark matter halos that appear in cosmological simulations, we obtain parameters $λ_{0}=0.04 \pm 0.005$ and $σ_λ=0.51 \pm 0.05$, interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of $λ$ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.