For decades, astronomers have been puzzled by the mysterious fate of the compact massive galaxies that dominated the early Universe.
These galaxies, once filled with trillions of stars, have all but vanished in modern times, leaving scientists scratching their heads about what happened to them.
We now know that these enigmatic objects were not destroyed or absorbed in galactic collisions, as previously thought.
Instead, they’ve been hiding in plain sight, cloaked in layers of stars.
The discovery of these “hidden” galaxies is a game-changer that could dramatically shift our understanding of galaxy evolution and our place in the cosmos.
A Glimpse into the Past: The Rise and Fall of Compact Galaxies
The early Universe was far different from what we observe today.
Instead of the vast, sprawling disc galaxies that dominate our skies, the early cosmos was populated by compact, elliptical galaxies—dense, star-packed systems that formed their stars much earlier than the galaxies we see today.
These compact galaxies were massive and contained trillions of stars, but as the Universe aged, they began to fade away.
Alister Graham, an astronomer at Swinburne University of Technology in Australia, has been researching these compact galaxies for years.
“When our Universe was young, there were lots of compact, elliptical-shaped galaxies containing trillions of stars,” Graham explains.
However, despite their initial abundance, such galaxies are practically non-existent in the present-day Universe.
In fact, when astronomers observe the sky today, they find very few galaxies that resemble the ones seen in the early Universe.
The Puzzle of the Vanishing Galaxies
Why did these massive galaxies seemingly disappear?
A widely accepted theory suggested that violent galactic mergers over time could have caused their destruction or transformation.
These mergers could have led to the formation of giant elliptical galaxies, which are much larger and more diffuse than the compact galaxies from the past.
However, there’s a problem with this theory: the number of galactic collisions required to account for such a drastic transformation just doesn’t add up.
There aren’t enough galactic collisions to explain the disappearance of these compact galaxies.
For years, astronomers assumed that compact galaxies had either merged into larger systems or simply vanished.
But new research suggests that the story is far more complex than we thought.
The Discovery of the “Hidden” Galaxies
In a breakthrough study led by Graham and his colleagues, the team has uncovered a surprising answer to the mystery.
It turns out that these compact galaxies didn’t vanish—they simply changed forms.
According to the study, the compact galaxies that once existed in the early Universe are still here today, but they’re hiding in plain sight, disguised by layers of stars and gas that make them appear much larger than they actually are.
The team discovered 21 compact spheroids—dense, star-packed regions—embedded in stellar discs.
These discs, which were previously overlooked by astronomers, made the galaxies appear much larger and prevented them from being properly identified.
In essence, these ancient galaxies had been “encased” in thin, disc-shaped star systems, making them appear paradoxically bigger and more diffuse than they actually were.
“They were hiding in plain sight,” says co-author Bililign Dullo in the study’s press release.
“The spheroids are cloaked by discs of stars that were likely built from the accumulation of hydrogen gas and smaller galaxies over the intervening eons.”
This discovery is a profound shift in our understanding of galaxy evolution.
Rather than disappearing through destruction or merger, these compact galaxies have simply been evolved into a different form.
The researchers suggest that the number of such hidden galaxies, when measured per unit volume of space, matches the number density of compact massive galaxies in the early Universe.
In other words, the “galactic dinosaurs” of our Universe are not extinct—they’ve merely been camouflaged by the formation of stars around them.
Why We Missed Them: The Problem with Galaxy Surveys
One of the key reasons these compact galaxies went unnoticed is the way modern astronomers conduct their surveys.
Traditional methods tend to treat individual galaxies as single entities, grouping the core bulge of stars and the outer disc together as one unit.
But in reality, these galaxies are far more complex than that.
While the bulge—the dense central cluster of stars—remains compact and massive, the outer disc can be much more spread out and diffuse.
As Giulia Savorgnan, a PhD student and co-author of the study, points out, “While the inner component is compact and massive, the full galaxy sizes are not compact.
This explains why they had been missed; we simply needed to better dissect the galaxies rather than consider them as single objects.”
By taking a closer look at the individual components of galaxies and separating the inner bulge from the outer disc, the team was able to uncover these long-hidden compact galaxies.
This new approach to galaxy observation is likely to lead to further discoveries of “missing” galaxies that have been previously overlooked by traditional methods.
A Dramatic Shift in Our Understanding of Galaxy Evolution
This study introduces a fundamentally different model of galaxy evolution that challenges our previous assumptions.
Instead of compact massive galaxies disappearing through violent mergers, the study suggests that they evolved into their present forms by growing stellar discs around their dense centers.
This process, which likely took billions of years, allowed these ancient galaxies to persist, albeit in a modified form.
The new findings suggest that these galaxies didn’t vanish—they adapted and evolved, and their compact cores still exist within the disc galaxies that we observe today.
This dramatic shift in our thinking has important implications for our understanding of how galaxies evolve over time, and how we define what a “galaxy” truly is.
As Graham states in the press release, “The galactic dinosaurs of our Universe are not extinct. They are simply embedded in large, relatively thin, discs of stars.”
Looking Ahead: What This Means for Future Research
This discovery opens the door to exciting new avenues of research in galaxy formation and evolution.
By re-examining how we classify and study galaxies, astronomers can gain a better understanding of how different types of galaxies emerge and evolve over time. Moreover, this shift in perspective may lead to the discovery of even more compact galaxies that have remained hidden from traditional surveys.
In the future, astronomers will likely use this new approach to revisit older data and perhaps uncover even more examples of these “hidden” galaxies.
By taking a more detailed look at the internal structure of galaxies, researchers may discover that many other compact systems are still lurking in the universe, waiting to be uncovered.
For now, the study has shed new light on the fate of the compact galaxies that once dominated the early Universe, showing that they have not disappeared but instead have evolved into a new form.
This research not only challenges long-held assumptions about galaxy evolution but also provides us with a deeper understanding of the vast and dynamic universe we live in.
The Future of Galactic Evolution
The revelation that these ancient galaxies have not vanished but rather evolved into a different form represents a crucial step forward in our understanding of the Universe.
As we continue to probe the depths of space, studies like these help us piece together the complex history of galaxies, offering valuable insights into how our own Milky Way and other galactic systems may have evolved over billions of years.
While the fate of the compact galaxies is now clearer, many questions remain.
What triggered the formation of their stellar discs? How did these transformations unfold over time?
And perhaps most importantly, what can these hidden galaxies teach us about the future of galaxy evolution in our own cosmic neighborhood?
As astronomers continue to refine their methods and explore the Universe with a more nuanced perspective, we can expect to uncover many more secrets about the past, present, and future of galaxies—secrets that may reshape our understanding of the cosmos for generations to come.
Sources:
- Swinburne University of Technology
- arXiv.org
- Astrophysical Journal
