Star forming filaments in warm dark matter models
  We performed a hydrodynamical cosmological simulation of the formation of a
Milky Way-like galaxy in a warm dark matter (WDM) cosmology. Smooth and dense
filaments, several co-moving mega parsec long, form generically above z 2 in
this model. Atomic line cooling allows gas in the centres of these filaments to
cool to the base of the cooling function, resulting in a very striking pattern
of extended Lyman-limit systems (LLSs). Observations of the correlation
function of LLSs might hence provide useful limits on the nature of the dark
matter. We argue that the self-shielding of filaments may lead to a thermal
instability resulting in star formation. We implement a sub-grid model for
this, and find that filaments rather than haloes dominate star formation until
z 6. Reionisation decreases the gas density in filaments, and the more usual
star formation in haloes dominates below z 6, although star formation in
filaments continues until z=2. Fifteen per cent of the stars of the z=0 galaxy
formed in filaments. At higher redshift, these stars give galaxies a stringy
appearance, which, if observed, might be a strong indication that the dark
matter is warm.
