Empirical distributions of galactic $\hat{I}»$ 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 $\hat{I}»$ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of $\hat{I}»$, 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 $\hat{I}»$ 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 $\hat{I}»$ 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 $\hat{I}{»}_0=0.04\pm 0.005$ and $\ddot{I}{ƒ}_{\hat{I}}»=0.51\pm 0.05$, interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of $\hat{I}»$ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.

8 pages, 6 figures, expanded final version, MNRAS (in press)