One in five distant galaxies remain hidden from our telescopes, camouflaged by cosmic dust

Researchers at the University of Copenhagen’s Niels Bohr Institute have discovered two previously invisible galaxies 29 billion light-years away from Earth. 

The two galaxies have been invisible to the optical lens of the Hubble Space Telescope, hidden behind a thick layer of cosmic dust that surrounds them. The two invisible galaxies suddenly appeared thanks to the giant ALMA radio telescopes (Atacama Large Millimeter Array) in Chile’s Atacama Desert, which can capture radio waves emitted from the coldest, darkest depths of the Universe.

“We were looking at a sample of very distant galaxies, which we already knew existed from the Hubble Space Telescope. And then we noticed that two of them had a neighbor that we didn’t expect to be there at all. As both of these neighboring galaxies are surrounded by dust, some of their light is blocked, making them invisible to Hubble,” explains Associate Professor Pascal Oesch of the Cosmic Dawn Center at the Niels Bohr Institute.

The new discovery suggests that the very early Universe contains many more galaxies than previously assumed. They simply lie hidden behind dust consisting of small particles from stars. However, they can now be detected thanks to the highly sensitive ALMA telescope and the method used by the researchers.

The two hidden galaxies are so far called REBELS-12-2 and REBELS-29-2. The light from the two invisible galaxies has traveled about 13 billion years to reach us. They are now located 29 billion light-years away due to the Universe’s expansion. Researchers used the ALMA telescope, which is based on radio signals.

The ALMA Telescope combines the light of all its 66 antennae to create a high-resolution image and spectra of the sky.

By comparing these new galaxies with previously known sources in the very early Universe, approximately 13 billion years ago, the researchers estimate that between 10 and 20 percent of such early galaxies may still remain hidden behind curtains of cosmic dust.

“Our discovery demonstrates that up to one in five of the earliest galaxies may have been missing from our map of the heavens. Before we can start to understand when and how galaxies formed in the Universe, we first need a proper accounting,” says Pascal Oesch.

NASA, ESA, and the Canadian Space Agency have built a new super telescope, the James Webb Space Telescope, which is expected to be launched into orbit on the 18th of December 2021. This Telescope will help in finding hidden galaxies.

With its strength and improved technology, the Telescope will gaze even deeper into the Universe and contribute new knowledge about its origins. 

“The next step is to identify the galaxies we overlooked, because there are far more than we thought. That’s where the James Webb Telescope will be a huge step forward. It will be much more sensitive than Hubble and able to investigate longer wavelengths, which ought to allow us to see these hidden galaxies with ease,” states Pascal Oesch, adding:

“We are trying to put the big puzzle about the universe’s formation together and answer the most basic question: ‘Where does it all come from?’ The invisible galaxies that we’ve discovered in the early Universe are some of the first building blocks of the mature galaxies we see around us in the Universe today. So that’s where it all began.”

Journal Reference

1. Fudamoto, Y., Oesch, P.A., Schouws, S. et al. Normal, dust-obscured galaxies in the epoch of reionization. Nature 597, 489–492 (2021). DOI: 10.1038/s41586-021-03846-z