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

Abstract

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