The origin of the correlation between the spin parameter and the baryon fraction of galactic disks

The puzzling correlation between the spin parameter λ of galactic disks and the disk-to-halo mass fraction fdisk is investigated. It is shown that such a correlation arises naturally from large uncertainties in determining the virial masses of dark matter halos. This result indicates that halo properties derived from fits to observed rotation curves are still very uncertain. An analysis of λ versus fdisk as function of the adopted halo virial mass shows that for reasonable halo concentrations 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.