SHOALS · The Swift GRB Host Galaxy Legacy Survey: Understanding the Link between GRBs and Star Formation from the Nearby to the Early Universe

The Swift GRB Host Galaxy Legacy Survey: Understanding the Link between GRBs and Star Formation from the Nearby to the Early Universe

The Universe has changed tremendously over thirteen billion years of cosmic history. Shortly after the Big Bang the cosmos was a nearly-featureless ocean of hydrogen and helium: the development of structure and heavy elements, prerequisites for the formation and evolution of planets and life, required much longer time scales. To understand our origins we would like to observe how this process unfolded across time. Astronomers do this by exploiting the vast size of the universe and the finite travel time of light: by observing galaxies billions of light-years away we see how the universe appeared billions of years ago. But this is difficult: galaxies at these vast distances appear very dim, making them exceedingly difficult to study in detail. To get around this difficulty, the SHOALS project relies on a tracer much more luminous than a typical galaxy whose intrinsic power can compensate for the effects of cosmic dimming. Long-duration gamma-ray bursts (GRBs) are produced by the explosions of rare, ultra-massive stars inside galaxies, and for a few minutes these events produce brilliant light at all electromagnetic wavelengths which can be studied in detail to reveal the presence, distance, chemical composition, and other properties of the galaxy in which it occurred. Even galaxies that would otherwise be too faint to be detected by the most powerful existing telescopes can be revealed by this technique. In this way, I and my collaborators pin-pointed a population of 119 galaxies which hosted GRBs during the past decade. We used information provided by observations of the burst while it was bright, as well as observations of the galaxy alone after the burst had faded, acquired via a multi-year, multi-facility observational campaign. Our immediate objective has been to build a catalog of the properties of GRB-sampled galaxies spanning the Universe's history, and use this catalog to accomplish several scientific objectives. First, we sought to better understand the composition of the universe at each epoch (such as the numbers of small, faint galaxies versus the number of large, fully-formed galaxies) and infer how rapidly this changed over the Universe's history. Second, we have attempted to better understand how well galaxies selected by our GRB technique match the galaxies selected by traditional astronomical surveys, in order to assess the advantages and disadvantages of each method. In particular, we have aimed to understand whether GRBs happen specifically in certain types of galaxies while avoiding others, perhaps because the production of the types of stars that produce these bursts requires particular conditions (such as a low concentration of heavy elements).

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