Introducing ‘UFO’ galaxies—the Milky Way’s dustier cousins

Banner image: A trio of "UFO" galaxies spotted by NASA's James Webb Space Telescope. (Credit: Nelson et al., 2023, The Astrophysical Journal Letters)

In a new study, a team of astrophysicists led by CU Boulder has set out to unravel the mysteries of UFOs-not the alien spacecraft, but a class of unusually large and red galaxies that researchers have nicknamed Ultra-red Flattened Objects, or UFOs for short.

The research shines a spotlight on some strange galaxies, said Justus Gibson, lead author of the study and a doctoral student in the Department of Astrophysical and Planetary Sciences. CU Boulder researchers first discovered UFO galaxies in images from the James Webb Space Telescope (JWST).

Now, Gibson and his colleagues think they know more about the galaxies' inner workings.

The researchers explained that UFOs are odd cosmic ducks for various reasons. For starters, they reside near the limit of how far earlier space instruments, like the Hubble Space Telescope, could peer into the universe. But Hubble had completely missed them because these galaxies emit very little visible light.

The new study relies on observations from the Webb telescope, a pioneering spacecraft that launched in December 2021. Drawing on those images and computer simulations, the team reports that UFO galaxies seem to be similar in size and shape to the Milky Way. But these new galaxies are much dustier.

The team published its findings this October in The Astrophysical Journal.

"JWST allows us to see this type of galaxy that we never would have been able to see before," Gibson said. "It tells us that maybe we didn't understand the universe as well as we thought."

The universe is turning out to be more interesting than some scientists assumed, said study co-author Erica Nelson, who first discovered the UFO galaxies.

"They're so visually striking," said Nelson, assistant professor of astrophysics at CU Boulder. "They're enormous red discs that pop up in these images, and they were totally unexpected. They make you say: 'What? How?'"

Hidden galaxies

Gibson noted that UFO galaxies look red because they emit very little visible light-most of the light that escapes these galaxies is infrared radiation, and what little visible light they emit is at the limit of what human eyes can see (red, in other words). As a result, the UFO galaxies were all but invisible to Hubble, which only records visible light. The Webb telescope, in contrast, collects infrared light, which means it's well-suited to spotting these kinds of objects.

"Prior to the launch of James Webb, we thought we would find really, really far away galaxies," Gibson said. "But we thought that closer to us, we already had a pretty good understanding of all the types of galaxies there are."

In the new study, Gibson and his colleagues drew on observations from a collaboration called the JWST Advanced Deep Extragalactic Survey (JADES). In all, the team identified 56 UFO galaxies in images from JADES.

They found a lot of dust.

Biting the dust

The researchers noted that all galaxies, and even Earth's solar system, contain interplanetary dust, the remnants of dying stars that exploded a long time ago, shooting tiny particles of metal far into space. But the UFO galaxies contain a lot more dust than the Milky Way-enough dust to block about 50 times more light from beaming into space. It's a bit like a sandstorm on Earth obscuring the sun.

The researchers also used computer simulations, or models, to understand how the galaxies are shaped. Gibson noted that galaxies can come in many shapes and sizes, from Frisbee-like discs to football shapes and spheres.

The team's calculations suggest that UFO galaxies may be shaped like run-of-the-mill discs (think Milky Way).

"You have these big bad disks-like our home, the Milky Way-flying around space, completely invisible to us," Nelson said.

How these galaxies got so dusty isn't clear. Nelson said she hopes that by studying them, astrophysicists can learn how galaxies grow and form new stars over time. For now, the UFOs raise a lot more questions than answers.

"Why on Earth do these galaxies have so much more dust than all the other galaxies?" she said. "Got me."

Other co-authors on the new study include researchers from the NSF's National Optical-Infrared Astronomy Research Laboratory, University of Pittsburgh, University of Massachusetts, Stanford University, Max Planck Institute for Astronomy, Harvard & Smithsonian Center for Astrophysics, University of Oxford, University of Cambridge, European Space Agency, University of Melbourne, Sorbonne University, University of Hertfordshire, University of Arizona, The Johns Hopkins University, University of California, Santa Cruz and NRC Herzberg.

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