They could shed light on the formation of heavy metals in the cosmos
About 30% of short gamma-ray bursts (sGRBs), which form in neutron star collisions, have no corresponding host galaxy, raising questions about their true origins and distances. New research has shown that they occur in extremely distant rare galaxies, in particular up to 10 billion light-years from Earth.
An international team of astronomers has succeeded in revealing that some short gamma-ray bursts (sGRB) they do not originate in diffuse areas of intergalactic space as previously thought, but in specific galaxies. These gamma rays were not “nomadic” bursts: data obtained from several observatories revealed that they were in fact forming in galaxies particularly distant and, therefore, very faint and difficult to detect.
The scientists, led by Dr Brendan O’Connor, an astronomer from the University of Maryland and George Washington University in the United States, reported that these seemingly isolated sGRBs actually occur in galaxies up to 10 billion light years from our planet. The new study was recently published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).
Additionally, the finding suggests that short gamma-ray bursts may have been more common in the past than previously believed. As they are produced under the neutron star mergers and these in turn forge heavy elements, such as gold and platinum, it is likely that these metals were dispersed throughout the Universe much earlier than previously thought.
Also called “gamma ray outbreaks”, sGRBs are gamma-ray flashes directly associated with extremely energetic explosions in distant galaxies. According to specialists, these are the most luminous electromagnetic events that occur in the Universe. Why, then, could their origin not be detected in many cases?
According to a press release, a large number of sGRBs are found in bright, relatively nearby galaxies, but some appear to have no specific galactic home. Now, thanks to a huge set of data from different observatories, such as the twin Gemini telescopes, it has been possible to find the faint glow of the galaxies from which these gamma rays originated, which were simply too distant to be recognized before.
Two hypotheses and an answer
sGRBs with no apparent host galaxy were a deep mystery to astronomers. The team led by O’Connor developed two hypotheses: the first postulated that progenitor neutron stars they formed as a binary pair in a distant galaxy, then drifted together through intergalactic space, eventually merging billions of years later. This would explain the absence of a specific origin, resulting from the constant expansion of the Universe.
Not satisfied with this explanation, they developed a second hypothesis: they argued that the neutron stars that generated the gamma rays merged billions of light-years away in their home galaxies, which now appear extremely faint due to their distance from the Earth’s position. The analyzes and observations ended up confirming this second hypothesis, solving the mystery for the moment.
Formation of heavy metals
Moreover, another important aspect is that this result could help astronomers to better understand the chemical evolution of the Universe. The merging of neutron stars triggers a series of cascading nuclear reactions, which are necessary to produce heavy metalssuch as gold, platinum and thorium, among many others.
If these types of discoveries push back the cosmic timescale associated with neutron star mergers, it would mean that the young universe it was much richer in heavy elements than previously thought. Therefore, the heaviest elements of the periodic table may have been common from the earliest stages of the formation of the cosmos.
An in-depth study of short GRB host galaxies on z ∼ 0 – 2: implications for shifts, redshifts and environments. B. O’Connor et al. MNRAS (2022). DOI: https://doi.org/10.1093/mnras/stac1982