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It is likely that cold molecular clouds form at high redshift, beforegalaxies. Considering these cold clouds instead of hot homogeneous gas as themain baryonic component of proto-galactic halos may affect several issuesduring galaxy formation. In particular, the baryonic matter loses angularmomentum through dynamical friction on the dark matter halo. In numericalsimulations using hot gas for baryonic matter, this gives rise to the so-calledangular momentum problem. In this work, we study the dynamical friction exertedon cold fractal gas by a collisionless background (dark matter) throughhigh-resolution numerical simulations. First, we find that, for values of theparameters relevant during galaxy formation, the friction does not alter themorphology of the fractal, which is mainly driven by internal dynamics. Then,we show that the presence of substructures and inhomogeneities in a body haslittle effect on the global value of the friction. Parameters such as branchingratio and fractal dimension also have little effect. In fact, we find out thatthe main effect comes from the deformations and fluctuations of the fractalstructure in cold gas clumps. If the deformation time is of the same order ofmagnitude or shorter than the typical build-up time of the friction, thefriction is weakened. We argue that this effect is relevant for galaxyformation, and that the angular momentum problem should not be worsened byincluding the small scale inhomogeneities of the cold gas which occur at aresolution out of reach of present simulations.Comment: 19 pages, 8 figures, Accepted for publication in A&

Dynamical friction on cold fractal gas clouds. Applications to disc formation