A recent study published in the Monthly Notices of the Royal Astronomical Society reveals the discovery of the most distant and most pristine fossil galaxy ever found, KiDS J0842+0059, located about 3 billion light-years away from Earth. This galaxy, which has remained largely unchanged for an astonishing 7 billion years, provides a unique opportunity to study the early universe and the processes that shaped the formation of massive galaxies.
The discovery was made possible by the work of a team led by the Italian National Institute for Astrophysics (INAF), who used cutting-edge tools like the Large Binocular Telescope in Arizona. The galaxy is not only the most distant fossil galaxy discovered so far but also the first of its kind observed beyond the local universe, a region only about 1 billion light-years from Earth. This find expands the frontiers of what we know about the early cosmos.
What Are Fossil Galaxies and Why Are They So Important?
Fossil galaxies are rare objects that provide a glimpse into a universe long past. They are galaxies that, unlike most others, escaped the typical evolutionary processes such as mergers and active star formation. After an initial, rapid burst of star formation, fossil galaxies stopped evolving. For reasons not yet fully understood, they remained undisturbed for billions of years.
“Relic galaxies, just by chance, did not merge with any other galaxy, remaining more or less intact through time,” explained Crescenzo Tortora, a researcher at INAF and lead author of the study. “These objects are very rare because, as time goes on, the probability to merge with another galaxy naturally increases.”
What makes fossil galaxies particularly fascinating is that they essentially serve as time capsules, preserving an image of the universe in its infancy. Studying these galaxies allows scientists to understand how galaxies first formed and how star formation occurred in the early stages of the universe. While most galaxies grow larger and merge with others, these fossil galaxies avoided such chaos, providing clues about the original conditions that led to their formation.
The Early Stages of Fossil Galaxy Formation
Fossil galaxies are born in two distinct phases. The first phase is marked by a sudden burst of star formation that is quick and violent. This intense period results in a galaxy that is compact, dense, and small — essentially the “progenitor” of the fossil galaxy.
“First, there’s an early burst of star formation, a very quick and violent activity,” said Chiara Spiniello, co-author of the study and researcher at the University of Oxford. “We end up having something very compact and small, the progenitor of this relic.”
The second phase is where the typical galaxy evolution takes place: galaxies grow larger by interacting with one another, merging and evolving their shapes and star populations. Fossil galaxies, however, missed this phase entirely. They formed almost entirely during the first star formation burst, and, as a result, they have a unique structure, with their mass nearly fully established in that first phase.
Why Are Fossil Galaxies So Dense and Unchanging?
One of the most defining features of fossil galaxies is their remarkable density and lack of star formation. Unlike our own Milky Way, which is still actively forming new stars, fossil galaxies have stopped producing new stars altogether. They contain billions of stars, each as massive as the sun, but they are static, doing “essentially nothing.”
“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,” said Spiniello. “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.”
This feature is what makes fossil galaxies such valuable objects of study. They have avoided the chaotic processes that most other massive galaxies have gone through, providing astronomers with a direct link to the conditions of the universe billions of years ago.
The Role of Supermassive Black Holes in Fossil Galaxies
Another aspect of fossil galaxies that remains a mystery is why they stopped forming stars so abruptly. Michele Cappellari, a professor of astrophysics at the University of Oxford, believes that feedback from supermassive black holes may be responsible for halting star formation in these galaxies. Supermassive black holes, found at the center of most large galaxies, can emit powerful winds that either expel or heat the gas within a galaxy, preventing the formation of new stars.
“As ‘living fossils,’ they have avoided the chaotic mergers and growth that most other massive galaxies have experienced. Studying them allows us to reconstruct the conditions of the universe in its infancy and understand the initial bursts of star formation,” said Cappellari. “Evidence from both local and distant observations suggests that feedback from supermassive black holes may be responsible. These black holes can produce powerful winds that expel or heat the gas in a galaxy, preventing further star formation. However, this remains an active area of research.”
What Makes KiDS J0842+0059 So Special?
KiDS J0842+0059 stands out even among other fossil galaxies for its extreme nature. It is what scientists call an “extreme relic” because almost all of its stars (99.5%) were formed during the initial starburst phase of the galaxy’s creation. After this burst, the galaxy has remained inactive, making it the perfect example of a fossil galaxy.
“This is what we call an extreme relic,” said Spiniello. “Because almost all, or 99.5% of its stars were formed incredibly early on in cosmic time, and the galaxy did absolutely nothing thereafter.”
This galaxy’s status as an extreme relic has important implications for our understanding of galaxy formation. It provides valuable information about the early universe and could help scientists understand why some galaxies evolve while others remain static.
The Future of Fossil Galaxy Research
The confirmation of KiDS J0842+0059 as a distant fossil galaxy is a major milestone in the study of the universe’s ancient past. As technology continues to advance, astronomers expect to discover more fossil galaxies, particularly with the help of telescopes like James Webb and Euclid, both of which are designed to observe distant objects in the universe. These instruments will make it easier to find and study these rare and mysterious objects, helping scientists further unravel the mysteries of the early universe.
“There must be something that prevents them from merging, but without knowing what, we cannot really predict what’s going to happen in the future,” said Spiniello, pointing out that while fossil galaxies have avoided mergers for billions of years, their future remains uncertain.
The Rarity of Fossil Galaxies
The rarity of fossil galaxies makes their study particularly important, but also challenging. The number of known fossil galaxies is very small, and it’s difficult to know exactly how rare they are. Sébastien Comerón, an astronomer at the Universidad de La Laguna in Spain, emphasized the rarity and mystery of these objects.
“Relic galaxies are mysterious,” Comerón said. “The fact that a few galaxies are nowadays untouched relics of the first large galaxies needs an explanation.”
The discovery of KiDS J0842+0059 gives scientists hope that more fossil galaxies can be identified and studied, offering a unique window into the early universe. By studying these ancient objects, astronomers may uncover vital clues about the birth of galaxies, star formation, and the evolution of the cosmos itself.