Spectroscopy of the environments of long gamma-ray bursts and their progenitors

Gamma-ray bursts (GRBs) are the most energetic explosions in the universe. Long-duration GRBs are thought to be caused by the final collapse of a massive star (ten to possibly a few hundred times the Sun). Massive stars, though they are much less numerous than lower-mass stars, greatly influence the structure and evolution of galaxies. Via their radiation, strong stellar winds and supernova explosions, massive stars dominate the energetic and chemical feedback to the interstellar medium. Metallicity, or the fraction of elements heavier than hydrogen and helium, is a central theme in this thesis. Although massive stars produce most of these metals, their evolution and fate also strongly depend on metallicity, as well as the production of GRBs. With state-of-the-art instrumentation, we study massive stars and their environments just beyond our Local Group of galaxies. Thanks to bright GRB afterglows, we can probe massive-star environments at cosmological distances and earliest times. The GRB afterglow acts as a backlight shining through its host galaxy; the spectrum of the host provides key information on, for example, its distance and the metallicity. Recently, we have been able to obtain high quality spectra of very distant GRBs, with redshifts between four and six. We used the powerful X-shooter spectrograph on the ESO Very Large Telescope, dedicated to study these faint and redshifted sources. This PhD thesis is a collection of spectroscopic studies of GRB afterglows and their massive-star progenitors.