Distributions of galactic lambda spin parameters: a comparison
between cosmological simulations and measurements from the SDSS
Xavier HERNANDEZ DORING
Instituto de Astronomia UNAM, Mexique
Résumé :
Using simple dimensional arguments for both spiral and elliptical
galaxies, we derive estimates of the halo spin parameter lambda for any
real galaxy, in terms of simple observational parameters. We apply this to
a large volume limited sample of galaxies taken from the SDSS data base.
The large numbers involved (11,597) allow the derivation of lambda
distributions with high accuracy. 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
lambda0=0.04 +/- 0.005 and
sigma of lambda =0.51 +/- 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.
A detailed comparison of correlations found between the observed
distributions (where a marked anti-correlation between lambda and mass is
found) and results from a high resolution N-body cosmological simulation
(where lambda and mass appear to be independent of each other) are also
discussed, highlighting the similarities and differences, as clues
regarding the angular momentum acquisition mechanism of galaxies. Recent
results concerning angular momentum correlations and alignment for pairs
of spirals will also be commented on.
