  We have performed TreeSPH simulations of disk galaxy formation in various
warm dark matter (WDM) cosmologies. Our results indicate that for a range of
WDM free-streaming masses, the disk galaxy formation angular momentum problem
can be completely resolved by going to the WDM structure formation scenario,
without having to invoke stellar feedback processes at all. We also confirm our
previous suspicion, that part of the angular momentum problem is due to
numerical effects, most likely related to the shock capturing, artificial
viscosity used in SPH. Furthermore we find that we can match the observed
I-band Tully-Fisher (TF) relation, provided that the I-band mass-to-light ratio
of disk galaxies is about 0.8. We argue that this is quite a reasonable value
in comparison with various dynamical and spectrophotometric estimates,
including one given in this paper. We speculate that our success in matching
the TF relation may be due to WDM halos being less centrally concentrated than
CDM halos and suggest to check this exciting possibility with high resolution
simulations, in particular in low Omega_M, WDM cosmologies. Finally, we discuss
possible physical candidates for WDM particles extensively. We find that the
most promising are neutrinos with weaker or stronger interactions than normal,
majorons (light pseudogoldstone bosons) or mirror or shadow world neutrinos.
