Lyman-Werner radiation delayed collapse of metal-free gas in the first galaxies

We investigate the process of metal-free star formation in the first galaxies with a high-resolution cosmological simulation. We consider the scenario in which a strong molecule-destroying Lyman-Werner (LW) background inhibits effective cooling in low-mass halos, delaying star formation until the collapse or more massive halos. Only when molecular hydrogen (H₂) can self-shield from LW radiation, which requires a halo capable of cooling by atomic line emission, will star formation be possible. To follow the formation of multiple gravitationally bound objects, at high gas densities we introduce sink particles which accrete gas directly from the computational grid. We find that in a 1 Mpc³ (comoving) box, runaway collapse first occurs in a 3×10⁷M_⊙ dark matter halo at z ≈ 12 assuming a background intensity of J₂₁ = 100. Due to a runaway increase in the H₂ abundance and cooling rate, a self-shielding, supersonically turbulent core develops abruptly with 10⁴ M_⊙ in cold gas available for star formation. We analyze the formation of this self-shielding core, the character of turbulence, and the prospects for star formation.