The Origin of the Correlation between the Spin Parameter and the Baryon Fraction of Galactic Disks

Abstract

The puzzling correlation between the spin parameter lambda of galactic disks and the disk-to-halo mass fraction fdisk is investigated. We show that such a correlation arises naturally from uncertainties in determining the virial masses of dark matter halos. This result leads to the conclusion that the halo properties derived from fits to observed rotation curves are still very uncertain which might explain part of the disagreements between cosmological models and observations. We analyse lambda and fdisk as function of the adopted halo virial mass. Reasonable halo concentrations require fdisk=0.01-0.07 which is significantly smaller than the universal baryon fraction. Most of the available gas either never settled into the galactic disks or was ejected subsequently. In both cases it is not very surprising that the specific angular momentum distribution of galactic disks does not agree with the cosmological predictions which neglect these effects.