This is the astro-ph blog of the Theoretical Modelling of Cosmic Structures group (TMoX) at the Max-Planck-Institute for Extraterrestrial Physics. We are an independent Max-Planck Research Group focusing on the various aspects in the formation and evolution of galaxies. Part of our focus is on the formation and evolution of early-type galaxies, super-massive black holes, the formation of the first structures in the universe and the enrichment history of the Universe. We are theoreticians using analytic modelling as well as numerical simulations in our work.

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16 April 2012

The Formation of the First Cosmic Structures and the Physics of the z ~ 20 Universe

Authors: O'Leary and McQuinn
Link: here

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.

1 comment:

  1. I really liked the thoroughness of checking the effects of the IC generation and the code comparisons. I find it very interesting that the SPH and AMR codes gave very similar results. I have two questions after reading the paper:

    1. Will a more generalized IC generator be made which properly includes the generation of a non-zero, non-homogeneous bulk velocity between the DM and baryons?

    2. With the introduction of the bulk flow in the baryons, a preferred direction is created. Will this be visible in the angular momentum and orientation of the first galaxies?