The mysterious galaxies, which were previously unknown to researchers, were discovered by a breakthrough new approach that allowed astronomers to look more deeply than ever before into the universe.
The astronomers describe the new find as a treasure trove, representing a huge set of galaxies. It could help solve some of the most deep and fundamental questions about the universe, including the mysteries of supermassive black holes and dark matter.
Some researchers had long thought that such hidden galaxies might be out in the universe, waiting to be found. But now they have finally been discovered and cosmologists will have to rethink their understanding of how the universe works.
Scientists got an unprecedented look at the universe when the Hubble Space Telescope was sent into space, and began looking around. But it could not see some of the most fundamental parts of what surrounds us.
The new research allowed scientists to tie together a variety of different observatories and use them to look more deeply than ever before and see the huge set of galaxies.
"This is the first time that such a large population of massive galaxies was confirmed during the first two billion years of the 13.7-billion-year life of the universe," said Tao Wang, from the University of Tokyo.
"These were previously invisible to us.
"This finding contravenes current models for that period of cosmic evolution and will help to add some details, which have been missing until now."
If you were able to see the galaxies themselves, they would be far more of a spectacle even than our own Milky Way.
"For one thing, the night sky would appear far more majestic," said Dr Wang, comparing the newly discovered part of the universe with our own.
"The greater density of stars means there would be many more stars close by appearing larger and brighter.
"But conversely, the large amount of dust means farther-away stars would be far less visible, so the background to these bright close stars might be a vast dark void."
But from Earth, the 39 galaxies are so difficult to see because they are so faint. Though they are the largest of their kind to be found, the light that reaches Earth is reduced and has been stretched because it has taken so long to travel to us.
"The light from these galaxies is very faint with long wavelengths invisible to our eyes and undetectable by Hubble," said Kotaro Koh, who worked on the new study published in Nature.
"So we turned to the Atacama Large Millimeter/submillimeter Array (ALMA), which is ideal for viewing these kinds of things.
"I have a long history with that facility and so knew it would deliver good results."
The stretching of the light can be a useful way for researchers to understand how far it has travelled, and therefore how old the galaxies are.
"It was tough to convince our peers these galaxies were as old as we suspected them to be," said Dr Wang.
"Our initial suspicions about their existence came from the Spitzer Space Telescope's infrared data.
"But ALMA has sharp eyes and revealed details at submillimeter wavelengths, the best wavelength to peer through dust present in the early universe.
"Even so, it took further data from the imaginatively named Very Large Telescope in Chile to really prove we were seeing ancient massive galaxies where none had been seen before."
In the future, we might even learn that the galaxies are more shocking than we realised. ""I'm eager for upcoming observatories like the space-based James Webb Space Telescope to show us what these primordial beasts are really made of," he said.
Researchers now hope to use the new galaxies to learn more about the universe itself. They will be look to understand the supermassive black hole that are at their middle, in the hope of learning more about how they form.
"Massive galaxies are also intimately connected with the distribution of invisible dark matter," said Professor Kohno. "This plays a role in shaping the structure and distribution of galaxies.
"Theoretical researchers will need to update their theories now."
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