The galaxies date from the early universe and have managed to escape our attention until now, because they were hidden behind a thick layer of dust.

Armed with the Atacama Large Millimeter/submillimeter Array (ALMA), however, researchers have now spotted the hidden galaxies — which formed more than 13 billion years ago. And it raises an interesting question: are there perhaps even more such ancient galaxies hidden in the universe?

Discovered by accident

Researchers weren’t even really looking for hidden galaxies. Instead, they used ALMA to study already known early galaxies. But at two of those early galaxies, in the seemingly empty space next to them, they stumbled upon a tentative indication that something was there after all. Subsequent observations then indicated that these were two early galaxies that are almost completely hidden by cosmic dust. One of the two will even go down in the books as the farthest dust-obscured galaxy scientists have discovered to date.

Not special, but dusty

Both galaxies formed more than 13 billion years ago. Other than that, they’re not really special. “These new galaxies have not been overlooked because they are extremely rare, but only because they were completely hidden by dust,” said study researcher Yoshinobu Fudamoto.

There may still be a lot to discover

And that is interesting. Because it is not often that researchers in the young universe come across such dusty galaxies. The fact that two have now been discovered in one fell swoop therefore raises the question of whether the astronomers were simply very lucky or whether galaxies obscured by dust are more common in the early universe than previously thought. In the latter case, it would mean that there are still quite a few galaxies waiting to be discovered. “We may have missed up to one in five galaxies in the young universe to date.”

Hubble and ALMA

The fact that the two galaxies have remained hidden from us for so long has everything to do with how galaxies from the young universe are studied. Almost everything we know about these galaxies to date is based on observations from the Hubble Space Telescope, which studies the ultraviolet emissions from these galaxies. Because dust absorbs UV emissions, Hubble cannot see galaxies hidden behind dust. Because ALMA observes at submillimetre wavelengths, it can detect the galaxies.

ALMA will undoubtedly be used in the future to find even more hidden galaxies. In addition, researchers also have high expectations of the James Webb Space Telescope to be launched at the end of this year. It is hoped that it – along with other instruments such as ALMA – can provide a more complete picture of the early galaxies and how they formed and evolved.