Formation of disk galaxies: warm dark matter and the angular momentum problem

We have performed TreeSPH simulations of disk galaxy formation in various w arm dark matter (WDM) cosmologies. Our results indicate that for a range of WDM free-streaming masses, the disk galaxy formation angular momentum prob lem can be considerably alleviated (and, we speculate, perhaps even complet ely resolved) by going to the WDM structure formation scenario, without hav ing to invoke stellar feedback processes. They also strongly suggest that p art of the angular momentum problem is due to numerical effects, most likel y related to the shock-capturing artificial viscosity used in smoothed part icle hydrodynamics (SPH). Furthermore, we find that we can match the observ ed I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is M/L-I similar or equal to 0.6-0.7. We argue that this is a fairly reasonable value compared to various dynamical and spectrophoto metric estimates, including two given in this paper. Finally, we extensivel y discuss possible physical candidates for WDM particles. We find that the most promising are neutrinos with weaker or stronger interactions than norm al, majorons (light pseudo-Goldstone bosons), or mirror- or shadow-world ne utrinos.