"The lynchpin of the survey is the Spitzer Space Telescope, which observes at mid-infrared wavelengths and is sensitive to the light from ""typical"" (older) stars and therefore can be used to measure a galaxy's total mass in stars. Using data from this telescope we were able to measure the stellar masses of all galaxies within the SHOALS sample and study how the mass distribution of GRB hosts evolves with time. We demonstrated that this distribution evolves relatively little from a redshift of z=5 (12.5 billion years ago) to z=1.5 (9 billion years ago) and that throughout this period the majority of galaxies are relatively luminous. However, between z=1.5 and z=0 (9 billion years to the present) the mass distribution changes rapidly: the largest and most massive galaxies disappear. We attribute this to sensitivity of the GRB progenitor to the presence of heavy elements (""metals""), which build up in galaxies over time. Specifically, we demonstrate that if gamma-ray bursts cannot occur in galaxies with a mean metal abundance higher than that of the Sun, both the mass distribution (and its time evolution) and the redshift distribution of our sample can be fully explained. These results were published in a pair of papers: Perley et al. 2016a (ApJ 817:7) describes the redshift distribution, and Perley et al. 2016b (ApJ 817:8) describes the mass distribution.
In addition to studying stellar masses with Spitzer, we also seek to analyse other properties of the galaxies: for example, how rapidly they are forming stars, or how much interstellar dust they contain. This will enable us to study how these properties change with cosmic time, and will also help us test whether GRBs might occur more readily in some galaxies over others for reasons not related to metal content. This requires additional observations at different wavelengths other than those probed by Spitzer, and while a great deal of data had already been compiled at the start of the project, the the data set was not complete at that time. New observations carried out over the course of the project at the Keck, VLT, and Magellan optical telescopes have now completed this data set and analysis of these data is underway.
We also acquired observations of smaller, targeted subsets of the sample at millimeter wavelengths, radio wavelengths, and using near-infrared spectroscopy. So far we have completed two studies with these data: in Perley et al. 2017a (MNRAS 465:970) we show that most gamma-ray burst host galaxies have moderate total star-formation rates, while in Perley et al. 2017b (MNRAS 465:89) we show that a report in pre-existing literature of a GRB host galaxy with above-solar metallicity (which was in tension with our Spitzer result, above) was actually a misidentification with a foreground galaxy.
Beyond the immediate objectives of the survey, we also have been pursuing investigations of GRB hosts from other observational campaigns (beyond SHOALS) and other types of luminous explosion which might have cosmic utility (such as luminous supernovae). The PI has also remained active in identifying and studying new gamma-ray bursts that exploded during the project, joining the pan-European X-shooter GRB collaboration. The published works most directly relevant to the SHOALS core science case are given in the Publications list. In total, the PI has co-authored 39 refereed publications which appeared or were first submitted during the project, and many additional works are in preparation or have been submitted for publication."
