The angular momentum content of dwarf galaxies: new challenges for the theory of galaxy formation

We compute the specific angular momentum distributions of a sample of low mass disk galaxies observed by Swaters. We compare these distributions to those of dark matter haloes obtained by Bullock et al. from high resolution N-body simulations of structure formation in a ΛCDM Universe. We find that although the disk mass fractions are significantly smaller than the Universal baryon fraction, the total specific angular momenta of the disks are in good agreement with those of dark matter haloes. This suggests that disks form out of only a small fraction of the available baryons, but yet manage to draw most of the available angular momentum. In addition we find that the angular momentum distributions of disks are clearly distinct from those of the dark matter; disks lack predominantly both low and high specific angular momentum. Understanding these findings in terms of a coherent picture for disk formation is challenging. Cooling, feedback and stripping, which are the main mechanisms to explain the small disk mass fractions found, seem unable to simultaneously explain the disk’s angular momentum distribution. In fact, it seems that the baryons that make up the disks must have been born out of angular momentum distributions that are clearly distinct from those of ΛCDM haloes. However, the dark and baryonic mass component experience the same tidal forces, and it is therefore expected that they should have similar angular momentum distributions. Therefore, understanding the angular momentum content of disk galaxies remains an important challenge for our picture of galaxy formation.