The formation of the first cosmic structures and the physics of the z~20 Universe

Abstract

We perform a suite of cosmological simulations in the LCDM paradigm of the formation of the first structures in the Universe prior to astrophysical reheating and reionization (15<~z<200). These are the first simulations initialized in a manner that self consistently account for the impact of pressure on the rate of growth of modes, temperature fluctuations in the gas, and the dark matter-baryon supersonic velocity difference. Even with these improvements, these are still difficult times to simulate accurately as the Jeans length of the cold intergalactic gas must be resolved while also capturing a representative sample of the Universe. Our simulations support the finding of recent studies that the dark matter-baryon velocity difference has a surprisingly large impact on the accretion of gas onto the first star-forming minihalos (with masses of ~10^6 Msun). In fact, the halo gas is often significantly downwind of such halos and with lower densities, which delays the formation of the first stars in most locations in the Universe. We also show that dynamical friction plays an important role in the nonlinear evolution of the differential velocity, acting to erase this velocity difference quickly in overdense gas as well as sourcing visually-apparent bow shocks and Mach cones throughout the Universe. We use simulations with both the GADGET and Enzo cosmological codes to test the robustness of these conclusions. We find that particle coupling in GADGET between the gas and dark matter particles can result in spurious growth that mimics nonlinear growth in the matter power spectrum. In a companion paper, we use the simulations presented here to make detailed estimates for the impact of the dark matter--baryon velocity differential on redshifted 21cm radiation. The initial conditions generator used in this study CICsASS can be publicly downloaded.