By creating the most extensive catalog to date of high-energy radiation blasts called short gamma-ray bursts (SGRBs), astronomers have traced these mysterious emissions back to active star-forming galaxies in the distant early universe.
Astronomers examined 84 SGRBs — the brightest explosions in the universe — and pinpoint them to their galactic source of origin. They then examined the characteristics of 69 of those galaxies. The team found that 85% of the short gamma-ray bursts they studied originated in young galaxies that are actively forming stars. Additionally, the researchers discovered that the majority of these high-energy radiation bursts were emitted when the 13.8-billion-year-old universe was young.
"This is the largest catalog of SGRB host galaxies to ever exist, so we expect it to be the gold standard for many years to come," research leader and Northwestern graduate student, Anya Nugent, said in a statement. "Building this catalog and finally having enough host galaxies to see patterns and draw significant conclusions is exactly what the field needed to push our understanding of these fantastic events and what happens to stars after they die."
SGRBs last just a few seconds but are so powerful that they appear bright even from great distances, meaning that up close they must outshine the combined light from hundreds of billions of stars in their home galaxies.
Astronomers believe that these short bursts of high-energy radiation come from the merger of two neutron stars, which are stellar remnants left over when massive stars reach the end of their lives and their cores collapse under the influence of gravity.
This core-collapse process squashes a mass at least equivalent to that of the sun down into a space the width of a city here on Earth, around 12 miles (19 kilometers). The result is material so dense that a teaspoon scooped from a neutron star would weigh 4 billion tons.
When two of these exotic stellar remnants orbit each other, they can spiral together and merge, a violent process that triggers an SRGB. While SGRBs last just a few seconds, they leave an optical afterglow that can last for hours before fading below astronomers' detection levels.
These emissions are currently the only way to study the early universe's population of merging neutron stars, so this extensive new catalog could facilitate a better understanding these violent and powerful events and the galaxies in which they arise.
Neutron star binaries emerge in a variety of galaxies
The astronomers created their SGRB catalog by using sensitive instruments at several observatories on Earth including the W.M. Keck Observatory in Hawaii, the Gemini Observatories in Hawaii and Chile, and the Magellan Telescopes at Las Campanas Observatory in Chile. In addition, they incorporated data from space-based telescopes — the Hubble Space Telescope and NASA's retired Spitzer Space Telescope — to capture deep imaging and spectroscopy of some of the faintest galaxies identified as SGRB hosts.
With that wealth of data, the astronomers were able to trace to their host galaxies four times as many SGRBs as had been managed in the past. This significantly larger dataset allowed the team to better characterize SGRB parent galaxies.
The astronomers found that these galaxies can be either young and star-forming or old and approaching death, demonstrating that neutron-star binary systems can form in a variety of environments.
The findings also indicate that neutron-star binaries can be short-lived, quickly merging after their stars collapse or pair off. "We suspect that the younger SGRBs we found in younger host galaxies come from binary stellar systems that formed in a star formation 'burst' and are so tightly bound that they can merge very fast," Nugent said. "Long-standing theories have suggested there must be ways to merge neutron stars quickly, but, until now, we have not been able to witness them."
But other SGRBs seem to come from slower collisions, since the researchers also found older SGRBs in more distant and thus more ancient galaxies. The stars in those galaxies may have either taken longer to initially form a binary or been more widely spaced, the scientists posit.
In another surprise, the researchers found SGRBs blasted out far outside their host galaxies, almost as if they were "kicked out" of their galactic homes. This finding raises the question of how SGRBs were able to wander so far from their host galaxies.
17 years of short gamma-ray burst detection
NASA's Neil Gehrels Swift Observatory first discovered the afterglow of an SGRB in 2005; now, astronomers detect and pinpoint at most a few dozen of these high-energy emissions each year.
Thus far, scientists have tracked only one SGRB back to a particular merger of binary neutron stars. That event, dubbed GRB 170817A, was detected seconds after gravitational waves from the same neutron-star merger. The detection rate of SGRBs and gravitational waves will likely improve substantially in the future, however, strengthening the connection between mergers and SGRBs.
"In a decade, the next generation of gravitational-wave observatories will be able to detect neutron-star mergers out to the same distances as we do SGRBs today," Wen-fai Fong, an astrophysicist at Northwestern University, said in the statement. "Thus, our catalog will serve as a benchmark for comparison to future detections of neutron-star mergers."
As future gravitational-wave detectors such as the European Space Agency's (ESA) future space-based laser interferometer mission LISA (Laser Interferometer Space Antenna) hunt for these tiny ripples in spacetime launched by powerful cosmic events they won't be alone in searching for neutron-star mergers.
The James Webb Space Telescope (JWST), with its highly sensitive infrared observing capabilities, is ideally positioned to hunt the distant (and thus ancient) universe for merging neutron stars and to study the environments in which they occur.
"I'm most excited about the possibility of using JWST to probe deeper into the homes of these rare, explosive events," Nugent said in the statement. "JWST's ability to observe faint galaxies in the universe could uncover more SGRB host galaxies that are currently evading detection, perhaps even revealing a missing population and a link to the early universe."
For Fong, Nugent's advisor at Northwestern, the SGRB catalog is the culmination of a decade of research and represents an important moment that emphasizes the importance of legacy in science.
"I started observations for this project 10 years ago, and it was so gratifying to be able to pass the torch onto the next generation of researchers," Fong concluded. "It is one of my career's greatest joys to see years of work come to life in this catalog, thanks to the young researchers who really took this study to the next level."