Final Report Summary - TK-GRB-10 (Probing the early Universe with GRB afterglows) This project uses Gamma-ray bursts (GRBs) as tools to investigate star-formation and the conditions in galaxies in the early Universe. GRBs are cosmic, stellar explosions, that emit a typical amount of energy of 10^51 erg in gamma-rays on short time scales of 0.1 to 100 seconds. The prompt emission in gamma-rays is followed by a longer-lasting afterglow, which can be detected in all wavelength ranges from radio, optical, to X- and gamma-rays up to several days after the explosion. The large energy release and high luminosity of GRBs and their afterglows make them ideal probes for studying the early Universe and the cosmic evolution. Furthermore, GRBs are linked to the death of massive stars, and hence to star formation in the very early evolution of the Universe. GRBs light up distant galaxies undergoing a process of star formation, which would remain undetected otherwise. The main scientific objective of this project is to constrain properties of low-mass, star-forming galaxies in the early Universe, which are the most common galaxies but difficult to find with different strategies; a GRB pinpoints their location. This primary aim is subdivided into three training and research aims. The first science aspect is to work on optical afterglow spectroscopy with the newly commissioned X-shooter instrument at the VLT. An X-shooter spectrum of a GRB afterglow is a powerful diagnostic of the chemical conditions in the local environment and the interstellar material of the host galaxies of GRBs. The second aspect is based on multi-wavelength studies of GRB afterglows using ground- and space-based observatories. The aim is to constrain the dust content and the total metal column density along the line of sight towards the burst. The third aspect is investigations about the population of GRB host galaxies. Since the beginning of the project, the project researcher has successfully gained experience in X-shooter spectroscopy and now is considered an expert in the respective field. This is evident from three invited talks on the relevant science objective that the project researcher gave during the time of the project.Building on the newly-gained experience and knowledge, he has written independent software for data reduction and analysis, that have been the basis of several papers in refereed journals. These software tools are tailored to the specific needs of GRB research and provide rapid data reduction and analysis as well as superior signal-to-noise and data quality compared to the standard tools. The main scientific results have being published in four first author papers in well-respected astronomical journals (three in Astronomy and Astrophysics, one in the Astrophysical Journal).In detail, the project researcher has performed the first survey of GRB host galaxies that were missing from previous sample because of a dust obscuration bias. He showed that this bias leads to an underrepresentation of massive and luminous galaxies in previous GRB host samples. Although published only in October 2011, this publication has already received more than 50 citations, demonstrating the significant relevance of this finding to the specific research field. In addition, has published the first detailed analysis of a GRB host galaxy at z > 1 that includes a measurement of the galaxy's metallicity - a crucial ingredient for understanding GRB formation. He also played a vital part in the long-lasting effort of building a large, unbiased and homogeneously selected sample of GRB hosts. He has provided the analysis for the sample of X-shooter observed GRB hosts that was being published as the fifth paper in a series of papers describing the observations and scientific results of this sample. Finally, the researcher has analyzed and interpreted an X-shooter spectrum of a GRB afterglow at redshift z = 2.4. He discovered the signatures of molecular hydrogen in this spectrum; only the second secure detection of H2 in a GRB-DLA so far. He went on and used this data to explain the previous paucity of H2 detections.During the time of the project, the project researcher has further contributed to 22 accepted or published publications in refereed journals as a co-author, as well as five publications that are submitted and currently being reviewed. These publications also mostly focused on the science objectives of the project, and/or often used the software tools developed by the researcher in the course of the project.
