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Astronomers discover ‘fossil galaxy’ 3 billion light-years away 
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A galaxy that has remained unchanged for 7 billion years — a rarity in the universe — has been observed by astronomers, offering a glimpse into cosmic history and adding to an enigmatic collection of objects called relics or “fossil galaxies.”

These space oddities are galaxies that, after an initial phase of intense star formation, escape their expected evolutionary path. While other galaxies expand and merge with one another, the fossil galaxies remain virtually inactive. Like celestial time capsules, they provide a snapshot into the ancient universe and allow astronomers to examine the mechanism of galaxy formation.

The newly discovered fossil galaxy — named KiDS J0842+0059 — is about 3 billion light-years from Earth, making it both the most distant and the first of its kind observed outside the local universe, the region of space closest to Earth that is approximately 1 billion light-years in radius. It was found by a team of astronomers led by the Italian National Institute for Astrophysics (INAF), using high-resolution imaging from the Large Binocular Telescope in Arizona.
“Relic galaxies, just by chance, did not merge with any other galaxy, remaining more or less intact through time,” said Crescenzo Tortora, a researcher at INAF and first author of a study on the finding published May 31 in the journal Monthly Notices of the Royal Astronomical Society. “These objects are very rare because, as time goes on, the probability to merge with another galaxy naturally increases.”

Very compact, very massive
Astronomers believe that the most massive galaxies form in two phases, according to study coauthor Chiara Spiniello, a researcher at the University of Oxford in the UK.

“First, there’s an early burst of star formation, a very quick and violent activity,” she said. “We end up having something very compact and small, the progenitor of this relic.”

The second phase, she added, is a protracted process during which galaxies that are in close proximity start interacting, merging and eating each other, causing a very dramatic change in their shapes, sizes and star populations. “We define a relic as an object that missed almost completely this second phase, having formed at least 75% of its mass in the first phase,” Spiniello explained.

The telltale feature of fossil galaxies is that they are very old, compact and dense, much more so than our own galaxy.

“They contain (billions) of stars as massive as the sun and they are not forming any new stars — they’re doing essentially nothing, and they are the fossil records of the very ancient universe,” she said. “They formed when the universe was really, really young. And then, for some reasons that we honestly don’t understand yet, they did not interact. They didn’t merge with other systems. They evolved undisturbed, and they remained as they were.”

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