Gamma-ray bursts—the brightest explosions in the universe—are acting as cosmic flashlights to illuminate the earliest galaxies and the gas between them. These bursts mark the deaths of massive stars in the young universe, and their afterglows let astronomers probe the chemical elements, dust, and radiation that built the first galaxies. A key puzzle is how the universe transitioned from a neutral, opaque state to the ionised, transparent one we see today. Stars in small, metal-poor galaxies are thought to have driven this "reionisation," but early data from gamma-ray burst sightlines suggest that ionising radiation may struggle to escape those galaxies—a finding that challenges the standard picture. The problem is that only a handful of high-redshift bursts have been studied in detail. This project will exploit a new wave of burst detections from the SVOM and Einstein Probe satellites, alongside follow-up with JWST and ground-based telescopes. By expanding the sample of well-studied high-redshift bursts, the team will test whether stars alone could have reionised the universe, or whether other sources are needed. This is fundamental science: it addresses how the cosmos evolved from darkness to light, with no immediate practical application, but such work has historically reshaped our understanding of physics and cosmology.
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Long-duration Gamma-ray bursts (GRBs) are highly luminous explosions that can be detected even at very high redshifts. They mark the end points of some massive stars, and so signpost the location of early star formation and star forming galaxies during, and potentially before, the era of reionization. Their bright power-law afterglows also provide ideal backlights with which to explore gas-phase abundances and kinematics, allowing us to map the build-up of chemical elements and dust (the legacy of earlier generations of stars), and address crucial questions including the average escape fraction of ionizing radiation and the neutral fraction of the intergalactic medium. We are about to enter a new era with enhanced rates of high-z GRB discovery thanks to the new SVOM and EP satellites operating along side Swift, and improved follow-up thanks to new instruments such as the Gemini/SCORPIO, NOT/NTE in addition to JWST. My groups have pioneered the discovery and exploitation of high redshift GRBs to study early galaxy populations, crucially including hosts that in some cases are likely below the sensitivity limits of even JWST. I have also highlighted the issue of the low average escape fraction of Lyman continuum radiation on GRB sight-lines, which appears problematic for the hypothesis of reionization by stars if GRB locations are typical of those of massive stars more generally, and if their hosts are typical of the small, low metallicity galaxies usually thought to have driven reionization. However, these conclusions are limited by the small sample sizes obtained to-date, motivating a concerted effort to increase the number of high-z GRBs with good spectroscopy and host studies during the finite life-times of these missions. This programme will build on the foundation of our previous work and leading role in GRB follow-up studies, to exploit this unique window on the high redshift universe, a window that provides a very complementary viewpoint compared to traditional galaxy studies.
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