Christa Gall – Niels Bohr Institute - University of Copenhagen

Christa Gall

Title: Genesis and evolution of dust in the early Universe

A thesis submitted July 30, 2010 for the degree of Doctor of Philosophy and defended October 8, 2010.

The Niels Bohr Institute, Dark Cosmology Centre, The Graduate School of Science, Faculty of Science University of Copenhagen Denmark.

Assoc. Prof. Anja C. Andersen

Prof. Jens Hjorth

Dr. Loretta Dunne
Dr. Hiroyuki Hirashita

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Genesis and evolution of dust in the early Universe

The most fascinating aspect of studying dust is the fact that small dust particles of a few micrometer which we cannot see with our naked eyes are a fundamentally important component in a Universe whose dimension we hardly can imagine. Dust grains impact the evolution of the Universe in many ways. For example they are known as the main formation site of molecular hydrogen which acts as important coolant by the formation of stars similar to our Sun. Dust is essential for the formation of planets and plays an important role in the end stages of life of most stars.

Large amounts of dust have been discovered in quasars (QSOs) at high redshift where the epoch of cosmic evolution was ! 1 Gyr, but the origin and evolution of these remains elusive. Supernovae (SNe) and asymptotic giant branch (AGB) stars have
been contemplated as prime dust sources due to their potential ability of generating sufficiently high amounts of dust. Though AGB stars are in fact known as the main dust source in the present Universe, their partially (too) long lifetimes questions their significance as dust contributers in the early Universe. SNe are sufficiently short-lived, but there exists a discrepancy between observationally and theoretically ascertained dust yields.

The principal aim of this thesis is to elucidate the astrophysical conditions required for generating these large amounts of dust in massive starburst galaxies and QSOs at high redshift. We first intend to identify the mass ranges of the most efficient dust producing stars at high redshift. We ascertain the dust production efficiency of stars in the mass range 3-40 M! using observed and theoretical dust yields of AGB stars and SNe. Based on these efficiencies we determine the total dust productivity for different stellar sources and investigate its dependency on the initial mass function (IMF). It is found that the dust production efficiency generally decreases with increasing progenitor mass. The total dust production strongly depends on the assumed IMF. AGB stars dominate the dust production if SNe produce " 3 × 10-3 M! of dust whereas SNe dominate if they are more efficient. The mass ranges of 8-12M! and 12-20 M! for SNe are equally important and dominate the overall SN contribution regardless of the IMF.

A main part of the thesis is devoted to the development of a numerical galactic chemical evolution model. The model is constructed such that the effect of a wide range of parameters can be investigated. We ascertain the temporal progression of the xv dustmass, the dust-to-gas and dust-to-metalmass ratios aswell as other physical properties of a galaxy and study their dependence on the mass of the galaxy, the IMF, dust production efficiencies and the degree of dust destruction in the interstellar medium (ISM). From this study we find that the amount of dust and the physical properties of a galaxy strongly depend on the initial gas mass available. Overall, while the total amount of dust produced increases with galaxy mass, the detailed outcome depends on the SN dust production efficiency, the IMF and the strength of dust destruction in the ISM. Dust production with a dominant contribution by AGB stars is found to be insufficient to account for the dust masses in excess of 108 M! within 400 Myr after starburst.

Furthermore, we investigate the influence of the star formation rate (SFR) of the starburst on the evolution of various quantities such as the amount of dust and gas, stellar masses, SFRs and the metallicity. We aim to determine the earliest epochs at which an agreement with observationally derived physical properties of QSOs at z ! 6 can be achieved. We apply the obtained results to individual QSOs at z ! 6. We find that large quantities of dust can be generated rapidly as early as 30 Myr after the onset of the starburstwhen the SFR of the starburst is ! 103 M! yr-1. The amount of dust and several physical quantities of individual QSOs at z ! 6 are satisfactorily reproduced by models at epochs 30, 70, 100 and 170 Myr and for galaxies with initial gas masses of 1-3 × 1011 M!. The best agreement with observations is obtained with top-heavy IMFs. A dominant dust contribution from SNe is required and a moderate dust destruction in the ISM can be accommodated, while at these epochs dust production by AGB stars is negligible.

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