Gamma-ray burst host galaxies at high resolution **time change! – Niels Bohr Institute - University of Copenhagen

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Gamma-ray burst host galaxies at high resolution **time change!

Christina Thöne, PhD defence

Gamma-ray bursts are the most powerful explosions in the universe and a very interesting phenomenon in themselves. Even though they are the brightest objects observed, they are nevertheless only stellar explosions or mergers of compact objects. This thesis does not deal with the GRB phenomenon itself but studies their environment and host galaxies with optical spectroscopy and using them as lightsources to study the distant universe. The first part presents three GRBs at different redshifts that allowed to study their host galaxies with absorption lines caused by material in the line of sight inside the host galaxy, namely GRB 060526, GRB 030329 and GRB 060206. For GRB 060526, we studied and modeled the afterglow of the GRB to determine its nature and observed the host galaxy with low resolution spectra to determine the metallicity. For GRB 030329 and GRB 060206 at the low and high end of the redshift distribution, the kinematics of the absorption lines indicate that those galaxies might have massive starburst winds as observed for some nearby galaxies like M 82. In the second part of the thesis, two hosts at very low redshift are presented that allow to resolve the stellar populations and study their properties with emission line spectroscopy. The host of GRB 060505 and the properties of the star-forming region around the burst site were especially interesting as the duration and therefore the progenitor of this GRB was highly debated. Through our study, we concluded that it was most likely a long-duration GRB and hence coming from the collapse of a massive star. NGC 2770 finally was the host of 3 Type Ib supernovae, which is unprecedented so far. Surprisingly, the galaxy itself has no extraordinary properties in terms of enhanced star formation or metallicity as suggested by theoretical models, also at the actual supernova sites. By comparing NGC 2770 with other galaxies with frequent SN occurrence, we conclude that the occurrence of 3 Ib supernovae were likely a chance coincidence.

Supervisor: Johan P.U.Fynbo/Dark Cosmology Centre

PhD-defence committee: Anja C. Andersen/Dark Cosmology Centre, Chair. Opponents: Chryssa Kouveliotou/NASA MSFC and Max Pettini/Institute of Astronomy, Cambridge

Thesis can be found here .