Environment and mass dependencies of galactic $λ$ spin parameter: cosmological simulations and SDSS galaxies compared

Abstract

We use a sample of galaxies from the Sloan Digital Sky Survey (SDSS) to search for correlations between the $λ$ spin parameter and the environment and mass of galaxies. In order to calculate the total value of $λ$ for each observed galaxy, we employed a simple model of the dynamical structure of the galaxies which allows a rough estimate of the value of $λ$ using only readily obtainable observables from the luminous galaxies. Use of a large volume limited sample (upwards of 11,000) allows reliable inferences of mean values and dispersions of $λ$ distributions. We find, in agreement with some N-body cosmological simulations, no significant dependence of $λ$ on the environmental density of the galaxies. For the case of mass, our results show a marked correlation with $λ$, in the sense that low mass galaxies present both higher mean values of $λ$ and associated dispersions, than high mass galaxies. This last direct empirical result, at odds with expectations from N-body cosmological simulations, provides interesting constrain on the mechanisms of galaxy formation and acquisition of angular momentum, a valuable test for cosmological models.