Exploring the environments of long-duration gamma-ray bursts.

摘要

Long-duration gamma-ray bursts (LGRBs) are the signatures of extremely energetic phenomena occurring throughout our universe. These events, commonly thought to be associated with the deaths of massive stars, have been proposed as possible tracers of star formation at high redshift; however, such an association is dependent on a thorough understanding of LGRB host environments and progenitors. In particular, the metallicity of LGRB host galaxies has become a matter of hot debate in recent years, with several studies suggesting that these events may be biased towards low-metallicity environments. The main goal of this dissertation is to perform the first in-depth study of the ISM environments and host galaxies that produce LGRBs. I have conducted the first dedicated spectroscopic survey of LGRB host galaxies, and used these observations along with data from the literature to determine a wide range of ISM properties for 16 z < 1 LGRB hosts and compare them to the general star-forming galaxy population. This work constructs the first mass-metallicity relation determined for LGRBs out to z ∼ 1. I have also generated an extensive suite of new stellar population synthesis and photoionization models, tailored towards modeling the host environments of LGRBs---these models show key improvements over past work, but also highlight several shortcomings in current model codes that must be addressed in future studies. Finally, I have examined red supergiants in low-metallicity Local Group galaxies, a poorly-understood but critical mass-losing phase of massive stellar evolution. From this work, I have concluded that LGRBs do exhibit a trend towards lower-metallicity host environments. However, observations of high-metallicity LGRB host galaxies and a comparison of the energetic and environmental properties of LGRBs both demonstrate that the complex role metallicity plays in LGRB progenitor formation remains unclear. New generations of galaxy models and continued studies of massive stellar evolution in low-metallicity environments are both vital to improving our understanding of the progenitors and host galaxies that give rise to these enigmatic events.