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

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

2006-07-26

arxiv.org/abs/astro-ph/0607605v2

Mon.Not.Roy.Astron.Soc.375:163-170,2007

Astronomy and Astrophysics

Astrophysics

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

Scientific paper

10.1111/j.1365-2966.2006.11274.x

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulas to derive an estimate of the halo spin parameter $\lambda$ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of $\lambda$, 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 $\lambda$ 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 $\lambda$ 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 $\lambda_{0}=0.04 \pm 0.005$ and $\sigma_{\lambda}=0.51 \pm 0.05$, interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of $\lambda$ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.

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