Astronomers are puzzling over an ancient galaxy from the dawn of the universe - which suddenly went dark billions of years ago.
The galaxy, known as XMM-2599, formed an immense number of stars by the time the universe was only one billion years old.
But then it was suddenly snuffed out, and scientists are not sure why.
Before the universe had reached two billion years, XMM-2599 had managed to form a mass of more than 300 billion suns, wrote postdoctoral researcher Benjamin Forrest in the Astrophysical Journal.
At its peak, this monster galaxy gave birth to more than 1,000 solar masses a year. By comparison, the Milky Way forms about one new star a year.
Researchers were able to make the detailed measurements and calculate the distance using powerful spectroscopic observational equipment located at the Keck Observatory on Maunakea in Hawaii.
"What makes XMM-2599 so interesting, unusual, and surprising is that it is no longer forming stars, perhaps because it stopped getting fuel or its black hole began to turn on," said Gillian Wilson, a professor of physics and astronomy.
A question on scientists' lips is what will become of the galaxy now its heyday is over.
"We do not know what it will turn into by the present day," Professor Wilson continued.
"We know it cannot lose mass. An interesting question is what happens around it. As time goes by, could it gravitationally attract nearby star-forming galaxies and become a bright city of galaxies?"
The team behind the research have been granted additional time at the observatory to look into some of the mysteries that still remain around XMM-2599.
The research team found XMM-2599 formed more than 1,000 solar masses a year in stars at its peak of activity -an extremely high rate of star formation.
In contrast, the Milky Way forms about one new star a year.
"XMM-2599 may be a descendant of a population of highly star-forming dusty galaxies in the very early universe that new infrared telescopes have recently discovered," said Danilo Marchesini, an associate professor of astronomy at Tufts University and a co-author on the study.
The evolutionary pathway of XMM-2599 is unclear.
"We have caught XMM-2599 in its inactive phase," Wilson said. "We do not know what it will turn into by the present day. We know it cannot lose mass. An interesting question is what happens around it. As time goes by, could it gravitationally attract nearby star-forming galaxies and become a bright city of galaxies?"
Co-author Michael Cooper, a professor of astronomy at UC Irvine, said this outcome is a strong possibility.
"Perhaps during the following 11.7 billion years of cosmic history, XMM-2599 will become the central member of one of the brightest and most massive clusters of galaxies in the local universe," he said. "Alternatively, it could continue to exist in isolation. Or we could have a scenario that lies between these two outcomes."