The Shocking History of the Early Universe

In arxiv:1204.1344, Matt McQuinn and I explored how the bulk flow between the dark matter and gas in the early Universe impacted the formation of the first structures and stars during the cosmic Dark Ages.

In a companion paper (arxiv:1204:1345), we estimated how this velocity difference impacts the 21cm signal from z~20 by shock heating the Universe and modulating the light from the first stars. A few movies from these studies are seen below:


Two movies of a 0.3 comoving Mpc patch of the Universe made with simulations run with the GADGET code and our initial conditions generator. The movies start at z=100 and end at z=15, are linear in time, and show the log of the gas density. In the movie on the left, the gas has not been given a relative velocity with respect to the dark matter, whereas in the movie on the right the gas is moving rightward at a Mach number of 4. (This Mach number stays constant w/ time at the cosmic mean density owing to how temperature and velocity redshift.) About 5% of the Universe would have had Mach numbers greater than 4. These simulations are the first to include the velocity difference between the baryons and dark matter in a consistent manner. At early times, you see small fluctuations (w/ an RMS of ~ 10-3 in the baryons) which grow into order-unity fluctuations by the end of the movies. The shapes of structures are radically changed in the case in which the baryons are moving supersonically, being notably more filamentary (even at z=100).


The above movies pan across a very tiny 70 comoving kpc GADGET simulation (the z=20 snapshot) on the left and a 140 comoving kpc Enzo simulation on the right. (Both of these box sizes are small so that they suppress the growth of structure, highlighting the low-contrast shock fronts.) The gas is moving to the right in both simulations with Mach number 4, and you can see that there are cone-like shock fronts that appear, sourced by the gas moving supersonically past collapsed structures! These cones engulf the volume during the Dark Ages of the Universe. The mass in these cones is pulling on the dark matter, causing the gas and dark matter to come into the same frame via this dynamical friction. In our simulations, a significant fraction of the gas has decelerated into the dark matter frame because of this process by z=20 (including essentially all of the overdense gas).

Movie shows a montage of the most massive halos in the simulation (105- 106.5 Msun), in order of decreasing dark matter halo mass. Each panel is a 70 comoving kpc zoom-in slice through one of these halos in the 0.7 comoving Mpc GADGET (top panels) and Enzo (bottom panels) simulations. The center of each panel is at the gas density peak within the halo. The first stars in the Universe are expected to form in >~105 Msun halos, and note that the relative velocity of the baryons (moving to the right with the specified Mach number) has a large impact even on the most massive halos that are shown. A mach number of 1.9 is typical, but this Mach number fluctuated spatially on 10-100 comoving Mpc scales with standard deviation of approximately 1 (and a Maxwellian distribution).

The above is similar to the previous movie, but instead showing temperature and entropy in the Enzo simulation at z=20. The virial shock (and the morphology of the halos) is significantly impacted by the streaming baryons (again flowing to the right). We find that its effect is significant even in cases where the circular velocity of the halo is an order of magnitude larger than the relative velocity of the gas. These halos have circular velocities of 2 to 7 km s-1, and the streaming velocity of the baryons is 0.6 km s-1.