The Cosmological Angular Momentum Problem of Low-Mass Disk Galaxies

Abstract

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 lambda'=lambda_DM*(j_d/m_d) is determined, where lambda_DM is the dark halo spin parameter and j_d/m_d is the ratio between the specific angular momentum of the disk and that of the dark halo. The distribution of lambda' is in excellent agreement with cosmological predictions of hierarchical clustering if j_d/m_d=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 lambda' and the baryonic mass fraction m_d: lambda'=0.4 m_d**0.6. 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 m_d 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.