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Statistical properties of dark matter mini-haloes and the criterion for HD formation in the early universe

Sasaki, Mei

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Abstract

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.

Item Type: Dissertation
Supervisor: Klessen, Prof. Dr. Ralf S.
Date of thesis defense: 4 February 2015
Date Deposited: 10 Mar 2015 09:20
Date: 2015
Faculties / Institutes: The Faculty of Physics and Astronomy > Dekanat der Fakultät für Physik und Astronomie
Subjects: 520 Astronomy and allied sciences
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