Formation of Disk Galaxies: Warm Dark Matter and the Angular Momentum Problem

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Abstract
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 considerably alleviated (and, we speculate, perhaps even completely resolved) by going to the WDM structure formation scenario, without having to invoke stellar feedback processes. They also strongly suggest that part of the angular momentum problem is due to numerical effects, most likely related to the shock-capturing artificial viscosity used in smoothed particle hydrodynamics (SPH). Furthermore, we find that we can match the observed I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is M/LI 0.6-0.7. We argue that this is a fairly reasonable value compared to various dynamical and spectrophotometric estimates, including two given in this paper. Finally, we extensively discuss possible physical candidates for WDM particles. We find that the most promising are neutrinos with weaker or stronger interactions than normal, majorons (light pseudo-Goldstone bosons), or mirror- or shadow-world neutrinos.