Inhomogenous Primordial Baryon Distributions on Sub-Galactic Scales: High-z Galaxy Formation with WDM

Abstract

For the Warm Dark Matter (WDM) cosmological model the implications of strongly inhomogenous, primordial baryon distribution on sub-galactic scales for Big Bang Nucleosynthesis, Cosmic Microwave Background anisotropies and Galaxy Formation (including fully non-linear evolution to z=0) are discussed, and the inflationary theory leading to such distributions is briefly reviewed. It is found that Big Bang Nucleosynthesis is essentially unaffected relative to SBBN and that the change in recombination history at z~1500-700 relative to ``standard'' theory leads to differences in the anisotropy and polarization power spectra, which should be detectable by the Planck satellite provided systematic effects can be accounted for. Moreover, it is shown by fully cosmological, hydro/gravity simulations that the formation of galactic discs is only weakly affected by going from smooth to highly non-homogenous, initial baryon distributions. In particular, the final disc angular momenta at z=0 are as large as for the standard case and the ``disc angular momentum problem'' is solved to within a factor of two or better without invoking (hypothetical) energetic feedback events. A very desirable difference relative to the the standard WDM model, however, is that the on-set of star (and AGN) formation happens earlier. For the ``optimal'' free-streaming mass scale of M_f~1.5x10^11 h^-1 Msun the redshift of formation of the first stars increases from z_*=4-5 to >6.5, in much better agreement with observational data on high-redshift galaxies and QSOs. It will, however, not be possible to push z_* above ~10, because at higher redshifts the gas velocity field is nowhere compressive. Probing the ``dark ages'' will hence enable a direct test of this theory.