Inhomogeneous primordial baryon distributions on subgalactic scales: high‐ z galaxy formation with warm dark matter

For the warm dark matter (WDM) cosmological model, the implications of a strongly inhomogeneous, primordial baryon distribution on subgalactic scales for big bang nucleosynthesis (BBN), cosmic microwave background (CMB) 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 BBN is essentially unaffected relative to standard BBN, 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‐homogeneous, 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 2 or better without invoking (hypothetical) energetic feedback events. A very desirable difference relative to the standard WDM model, however, is that the onset of star (and active galactic nucleus) formation happens earlier. For the ‘optimal’ free‐streaming mass scale of M f ∼ 1.5 × 10 11 h −1 M ⊙ , the redshift of formation of the first stars increases from z ★, i = 4–5 to ≳6.5, in much better agreement with observational data on high‐redshift galaxies and quasi‐stellar objects. However, it will not be possible to push z ★, i above ≈10, because at higher redshifts the gas flow is nowhere compressive. Probing the ‘dark ages’ will hence enable a direct test of this theory.