The Cosmological Angular Momentum Problem of Low-Mass Disk Galaxies

The rotational properties of the visible and dark components of low-mass disk galaxies (vrot ≤ 100 km/s) are investigated using the Swaters sample. The rotational parameter λ = λ(jd/md) is determined, where λ is the dark halo spin parameter and jd/md is the ratio between the specific angular momentum of the disk and that of the dark halo. The distribution of λ is in excellent agreement with cosmological predictions of hierarchical clustering if jd/md ≈ 1, that is if the protogalactic gas did not loose a significant amount of specific angular momentum due to dynamical friction. This result is in disagreement with current cosmological Nbody/SPH simulations where the baryonic component looses 90% of its specific angular momentum while settling into the equatorial plane. The Swaters sample also shows a surprisingly strong correlation between λ and the baryonic mass fraction md: λ ′ = 0.4m d . This correlation can be explained if the total amount of gas in protogalaxies is a universal fraction of their dark matter mass, of order 10%, and if the variation in md is a result of the fact that only the inner parts of the primordial gas distribution managed to form a visible disk component. In this case the specific angular momentum of the gas out of which the disk formed is a factor of 2.75 larger than that of the dark halo, which would require a yet unknown spin-up process for the visible baryonic component. Subject headings: cosmology: theory – dark matter – galaxies: evolution – galaxies: formation – galaxies: structure