Statistical properties of dark matter mini-haloes andthe criterion for HD formation in the early universe
2015
The aim of this work is to explore how dark matter structures and astronomical objects
(the first generation of stars) formed in the high-redshift universe. We investigate
properties of dark matter mini-haloes and clarify the process of primordial star formation
that takes place in different dark matter mini-haloes. The specific questions that
we aim to answer in this work include how dark matter mini-haloes found at z >= 15
differ from their more massive lower-redshift counterparts and what determines the
amount of HD that forms in primordial gas at the initial stage of protostellar collapse.
We ran a high-resolution N-body simulation that has the highest mass resolution ever
achieved for a representative cosmological volume at these high redshifts, and made
precision measurements of various physical properties that characterise dark matter
haloes. As expected from the differences in the slope of the dark matter density
power spectrum, the dependence of formation time on dark matter halo mass is very
weak in the case of the haloes that we study here. Despite this difference, dark matter
structures at high redshift share many properties with their much more massive
counterparts that form at later times. We ran a separate set of cosmological hydrodynamical
simulations to study gas starting to collapse in dark matter haloes. We found
that in some of our simulated mini-haloes, HD cooling became important during the
initial collapse, and investigated in detail why this occurred. We compared HD-rich
and HD-poor mini-haloes in our simulations and found that the amount of HD that
forms is linked to the speed of the gravitational collapse. If the collapse is rapid,
dynamical heating prevents the gas from cooling to temperatures low enough for HD
cooling to become important, but if the collapse is slow, HD cooling can come to
dominate, resulting in a minimum gas temperature which is lower by a factor of two.
We investigated what properties of the mini-haloes were responsible for determining
the collapse time, and showed that, contrary to previous suggestions, the mass of the
mini-halo and the rotational energy of the gas appear to have little in
uence on the
speed of the collapse. We therefore suspect that the main factor determining whether
the collapse is slow or rapid, and hence whether HD cooling becomes important or
not, is the degree of turbulence in the gas.
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