About 30% of short gamma-ray bursts (sGRBs), which form during the collisions of neutron stars, lack a coincident host galaxy, raising questions about their true origins and distances. Using data from several space- and ground-based telescopes, astronomers have found that these seemingly isolated sGRBs actually occurred in remarkably distant galaxies up to 10 billion light-years away. This discovery suggests that sGRBs may have been more common in the past than expected. Since neutron-star mergers forge heavy elements, including gold and platinum, the Universe may have been seeded with precious metals earlier than expected.
“Many sGRBs are found in bright galaxies relatively close to us, but some of them appear to have no corresponding galactic home,” said Dr. Brendan O’Connor, an astronomer at the University of Maryland and the George Washington University.
“By pinpointing where sGRBs originate, we were able to comb through troves of data from observatories like the twin Gemini telescopes to find the faint glow of galaxies that were simply too distant to be recognized before.”
Dr. O’Connor and colleagues began their quest by reviewing data on 120 sGRBs captured by two instruments aboard NASA’s Neil Gehrels Swift Observatory: Swift’s Burst Alert Telescope, which signaled a burst had been detected; and Swift’s X-ray Telescope, which identified the general location of the sGRB’s X-ray afterglow.
Additional afterglow studies made at Lowell Observatory more accurately pinpointed the location of the sGRBs.
The afterglow studies found that 43 of sGRBs were not associated with any known galaxy and appeared in the comparatively empty space between galaxies.
“These hostless sGRBs presented an intriguing mystery and astronomers had proposed two explanations for their seemingly isolated existence,” Dr. O’Connor said.
One hypothesis was that the progenitor neutron stars formed as a binary pair inside a distant galaxy, drifted together into intergalactic space, and eventually merged billions of years later.
The other hypothesis was that the neutron stars merged many billions of light-years away in their home galaxies, which now appear extremely faint as a result of their vast distance from Earth.
“We felt this second scenario was the most plausible to explain a large fraction of hostless events,” Dr. O’Connor said.
“We then used the most powerful telescopes on Earth to collect deep images of the GRB locations and uncovered otherwise invisible galaxies 8 to 10 billion light-years away from Earth.”
This result could help astronomers better understand the chemical evolution of the Universe.
Merging neutron stars trigger a cascading series of nuclear reactions that are necessary to produce heavy metals, like gold, platinum, and thorium.
Pushing back the cosmic timescale on neutron-star mergers means that the young Universe was far richer in heavy elements than previously thought.
“This pushes the timescale back on when the Universe received the ‘Midas touch’ and became seeded with the heaviest elements on the periodic table,” Dr. O’Connor said.
The team’s paper was published in the Monthly Notices of the Royal Astronomical Society.
B. O’Connor et al. A deep survey of short GRB host galaxies over z ∼ 0 − 2: implications for offsets, redshifts, and environments. MNRAS, published online July 26, 2022; doi: 10.1093/mnras/stac1982
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