Master Thesis defense by Simone Vejlgaard Nielsen

Title: Characterising the Interstellar and Intergalactic Medium Surrounding a Gamma-Ray Burst at z ~ 6.3

Abstract: Understanding the composition of the interstellar (ISM) and intergalactic medium (IGM) within the first billion years after the Big Bang is a central topic in current astrophysics. In this thesis, I use the high signal-to-noise ratio, optical/near-infrared spectrum of a recently detected afterglow from the Swift gamma-ray burst (GRB) 210905A at redshift z ∼ 6.3 to gain new information on this early period of the Universe’s history. I detect metal absorption lines corresponding to a relatively high metallicity and dust-to-metals ratio for the redshift as well as an overabundance of aluminium and magnesium. The aluminium overabundance suggests that massive, rotating stars are present in the GRB host galaxy, while the high dust-to-metals ratio requires rapid dust production within this galaxy. Another interpretation is α-enhancement, which lowers both the metallicity and the dust-to-metals ratio. Further, I detect absorption from neutral hydrogen on the GRB continuum, which I fit using Importance Nested Sampling. The best fit contains a non-zero neutral hydrogen fraction in the IGM at z ~ 6.3, which has never before been directly measured in a GRB afterglow. This result provides a strong constraint on the Epoch of Reionisation because of its correlation with a late and rapid reionisation of the Universe.

Supervisors:

• Johan Fynbo, University of Copenhagen, Niels Bohr Insitute
• Kasper Elm Heintz, University of Copenhagen, Niels Bohr Insitute

Censor:

• Frank Grundahl, Aarhus University, Department of Physics and Astronomy