Castaway gamma-ray bursts occur in extremely distant galaxies, astronomers say

About 30% of short gamma-ray bursts (sGRBs), which form in neutron star collisions, have no coincident host galaxy, raising questions about their true origins and distances. Using data from multiple space and ground-based telescopes, astronomers found that these seemingly isolated sGRBs actually occur in remarkably distant galaxies up to 10 billion light-years away. This finding 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 sooner than expected.

This artist’s impression illustrates the merger of two neutron stars, which produces the remarkably brief but intensely powerful event known as a short gamma-ray burst. Image credit: NOIRLab / NSF / AURA / J. da Silva / Spaceengine.

“Many sGRBs are found in bright galaxies relatively close to us, but some of them appear to have no corresponding galactic focus,” said Dr Brendan O’Connor, an astronomer at the University of Maryland and the University of Maryland. Washington University.

“By pinpointing the origin of sGRBs, we were able to sift through troves of data from observatories like the twin Gemini telescopes to find the faint glow of galaxies that were simply too distant to recognize before.”

Dr O’Connor and his colleagues began their quest by examining data from 120 sGRBs captured by two instruments aboard NASA’s Neil Gehrels Swift Observatory: Swift’s Burst Alert Telescope, which reported that a burst had been detected; and the Swift X-ray Telescope, which identified the general location of the sGRB X-ray afterglow.

Additional afterglow studies performed at the Lowell Observatory further identified the location of the sGRBs.

Afterglow studies revealed that 43 of the sGRBs were not associated with any known galaxy and appeared in the relatively empty space between galaxies.

“These hostless sGRBs presented an intriguing mystery, and astronomers had offered two explanations for their seemingly isolated existence,” said Dr O’Connor.

One hypothesis was that neutron progenitor stars formed as a binary pair inside a distant galaxy, drifted together through intergalactic space, and finally merged billions of years later.

The other hypothesis was that neutron stars merged billions of light-years away in their home galaxies, which now appear extremely faint due to their great distance from Earth.

“We felt that this second scenario was the most plausible for explaining a large part of the hostless events,” Dr O’Connor said.

“We then used the most powerful telescopes on Earth to collect deep images of GRB locations and discovered otherwise invisible galaxies 8 to 10 billion light-years from Earth.”

This result could help astronomers better understand the chemical evolution of the Universe.

Merging neutron stars trigger a series of cascading nuclear reactions that are needed to produce heavy metals, such as gold, platinum and thorium.

Pushing back the cosmic timescale on neutron star mergers means the young Universe was much richer in heavy elements than previously thought.

“It pushes back the time scale when the Universe received the ‘Midas touch’ and seeded itself with the heaviest elements on the periodic table,” Dr O’Connor said.

The team’s article was published in the Royal Astronomical Society Monthly Notices.


B.O’Connor et al. An in-depth study of short GRB host galaxies on z ∼ 0 – 2: implications for shifts, redshifts and environments. MNRAS, posted on July 26, 2022; doi: 10.1093/mnras/stac1982

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