Copyright by Athena Ranice Stacy 2011 - The University of Texas at ...

hierarchical zoom-in procedure (see Stacy et al. 2010 for more details). We
added three additional nested refinement levels of 40, 30, and 20 kpc (comov-
ing), centered on the location where the first minihalo will form. At each level
of refinement, we replaced every ‘parent’ particle with eight ‘child’ particles,
such that at the highest refinement level each parent particle has been replaced
with 512 child particles. The highest-resolution gas particles have a mass
mSPH = 0.015 M⊙, so that the mass resolution is Mres 1.5NneighmSPH < ∼ 1
M⊙, where Nneigh 32 is the typical number of particles in the SPH smoothing
kernel (e.g. Bate and Burkert 1997).
3.2.2 Chemistry, Heating, and Cooling
We use the same chemistry and cooling network as described in Greif
et al. (2009). The code follows the abundance evolution of H, H + , H − , H2,
H + 2 , He, He + , He ++ , and e − , as well as the three deuterium species D, D + , and
HD. All relevant cooling mechanisms are accounted for, including H2 cooling
through collisions with He and H atoms and other H2 molecules. Also included
are cooling through H and He collisional excitation and ionization, recombi-
nation, bremsstrahlung, and inverse Compton scattering. We finally note that
H2 cooling through collisions with protons and electrons is included, as they
play an important role within H ii regions (Glover and Abel 2008).
One important difference we note between the high density (n > ∼ 10 9
cm −3 ) evolution in the current simulation and that of Stacy et al. (2010) is
that we here account for the optical thickness of the H2 ro-vibrational lines,
which reduces the effectiveness of these lines in cooling the gas. We include
this effect using the Sobolev approximation (see Yoshida et al. 2006; Greif
et al. 2011 for more details).
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