Star Formation in the First Galaxies I: Collapse Delayed by Lyman-Werner
  Radiation
  We investigate the process of metal-free star formation in the first galaxies
with a high-resolution cosmological simulation. We consider the cosmologically
motivated scenario in which a strong molecule-destroying Lyman-Werner (LW)
background inhibits effective cooling in low-mass haloes, delaying star
formation until the collapse or more massive haloes. 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 Mpc^3 (comoving) box, runaway collapse first occurs
in a 3x10^7 M_sun 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 ~10^4
M_sun 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. Due to a lack of fragmentation on scales we resolve, we argue
that LW-delayed metal-free star formation in atomic cooling haloes is very
similar to star formation in primordial minihaloes, although in making this
conclusion we ignore internal stellar feedback. Finally, we briefly discuss the
detectability of metal-free stellar clusters with the James Webb Space
Telescope.
