The Monthly Media for June 2022 comes from PhD student Sonja Panjkov, who discusses an exciting Astro3D paper on Gamma-Ray Bursts (GRBs) and where these highly energetic events occur. This research is led by University of Melbourne PhD student Benjamin Metha and his supervisor Prof Michele Trenti.

Gamma-ray Bursts, or GRBs, are the most energetic events in the Universe since the Big Bang, releasing approximately 1053 ergs of energy, equivalent to more than 1033 (a billion septillion) atomic bombs. However, the precise environments in which GRBs occur continue to perplex the astronomy community.

Luckily Benjamin and Michele have attempted to shed light on these powerful events using a state-of-the-art simulation of the Universe.

Artist’s conception of GRB 190114C. Credit: NASA, ESA & M. Kornmesser. 

Two possible channels have been suggested for the formation of GRBs. The first involves the core-collapse supernova of an isolated, fast-spinning star, dubbed the ‘collapsar’ model, while the second consists of the merger of two stars in a binary system.

One important galaxy property that can help us understand GRBs is metallicity. It tells astronomers what fraction of a galaxy is made up of metals, or elements heavier than hydrogen and helium. And it has previously been suggested to influence where GRBs occur.

More specifically, the single star collapsar model is predicted to occur in low metallicity galaxies, due to the reduced rotation of stars in high metallicity environments. On the other hand, the binary channel can occur in a variety of environments since the merger itself can generate the stellar rotation required for a successful GRB.

Interestingly, observational studies have revealed that GRBs occur in both low and high metallicity galaxies, leading several research groups to suggest that both the collapsar and binary formation mechanism are at play.

However, Benjamin and Michele propose an alternative solution: that high metallicity host galaxies contain smaller, low metallicity regions in which GRBs occur. As an example, think of a desert containing an oasis. The desert is very dry overall, however the pocket of area containing the oasis will likely contain a lot of water.

To investigate this scenario, they use the IllustrisTNG simulation which models galaxy formation to probe the internal metallicity variations of GRB host galaxies. They compare their predictions to observational results, and their findings are shown in the plots below.

The relative likelihood of a cutoff bias model for GRB formation, taking into account the internal metallicity variations (left) and using only the average galaxy metallicity (right). In these plots, the most likely metallicity threshold is determined by finding the maximum value of the curves. Therefore, in the left plot, the most likely threshold is 0.35 for all 3 sets of observational results. In the right plot, the most likely cutoff for each set of observations is different.

In the left plot, Benjamin and Michele test a model referred to as a cutoff bias function, in which GRBs only occur below some threshold metallicity. When the internal metallicity distribution of the GRB host galaxy is considered, they find a cutoff metallicity of 35% of the Sun’s metallicity as the most likely scenario. In addition, their predictions under this model reproduce all 3 sets of observations, including GRB rate, metallicity and mass data.

In the right plot, they instead try a model that ignores any internal metallicity variations, which leads to a different cutoff metallicity for each of the three sets of observations. As a good model should be able to reproduce all observations, it is clear that accounting for small-scale metallicity variations is key to understanding GRBs and their host galaxies.

Benjamin’s and Michele’s results go some way in answering where GRBs occur, however there is always more work to be done. With the recent release of the first images from the James Webb Space Telescope, better-resolved observations of GRB host galaxies will soon be possible, allowing researchers to learn even more about the environments in which GRBs occur. But that’s for another Monthly Media.