Authors: O'Leary and McQuinn
This paper focuses on three aspects of research into simulations of the early Universe. First, they properly (almost -- see first comment) solve the equations of linear growth of baryons and dark matter. They include four major additions to previous solutions. 1. The baryons and dark matter are solved separately, allowing the impact of pressure on the baryons to be properly included. 2. Radiation is included in the calculation of the initial densities and velocities. 3. The mean temperature (and fluctuations in T) and electron density are self-consistently included. 4. They include a (homogeneous) bulk velocity between the dark matter and baryons. The overall effect of these changes is best shown in Figure 15, where it is clear that for k = 30-2000 Mpc^-1, there is a significant difference in the growth factor of dark matter at z = 30. Previous IC generators (including previous works which included a velocity difference between DM and baryons) yielded too much power in this range.
The authors then took their improved IC generator and ran simulations using an AMR code (ENZO) and an SPH code (GADGET3). They provide detailed comparisons between the codes, as well as changing various aspects of the simulation (e.g. starting redshift, box size, & bulk flow velocity). These are best seen in Figures 4, 5, & 6). There is little difference between the two codes, with two small exceptions. In GADGET with no bulk flow, there is particle coupling between the DM and baryon particles which add spurious power (Fig 5). In Enzo with AMR, there is a small increase in power in the baryons due to the AMR grid size.
Lastly, they compare the formation of the first structures in a universe with a non-zero bulk flow to that of typical first-structure formation. The basic conclusion is two-fold. First, the effect of a bulk flow depends strongly on environment -- whether or not the halos is forming in filaments aligned or transverse with the bulk flow. Second, the wind means smaller total gas masses, less dense gas in halos, and less gas available for star formation. In a companion paper, the authors plan to discuss the impact a bulk flow will have on the 21cm line absorption and the growth of Lyman-Werner and other radiation backgrounds.
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