Galaxies in the Early Universe Were Shaped Like Bananas, Study Suggests

What does a newborn galaxy look like?

For the longest time, many astrophysicists and cosmologists have assumed that newborn galaxies would look like the orbs and spidery discs familiar in the modern universe.

But according to an analysis of new images from the James Webb Space Telescope, baby galaxies were neither eggs nor discs. They were bananas. Or pickles, or cigars, or surfboards — choose your own metaphor. That is the tentative conclusion of a team of astronomers who re-examined images of some 4,000 newborn galaxies observed by Webb at the dawn of time.

“This is both a surprising and unexpected result, though there were already hints of it with Hubble,” said Viraj Pandya, a postdoctoral fellow at Columbia University, referring to the Hubble Space Telescope. He is the lead author of a paper soon to be published in the Astrophysical Journal under the provocative title “Galaxies Going Bananas.” Dr. Pandya is scheduled to give a talk about his work on Wednesday at a meeting of the American Astronomical Society in New Orleans.

If the result holds, astronomers say that it could profoundly alter their understanding of how galaxies emerge and grow. It could also offer insight into the mysterious nature of dark matter, an unknown and invisible form of matter that astronomers say makes up a major part of the universe and outweighs atomic matter 5 to 1. Dark matter engulfs galaxies and provides the gravitational nurseries in which new galaxies arise.

The result builds on hints from earlier observations from the Hubble telescope that the earliest galaxies were shaped like pickles, said Joel Primack, an astronomer at the University of California, Santa Cruz, and an author of the new paper.

In an email, Alan Dressler of the Carnegie Observatories, who was not part of Dr. Pandya’s work, characterized the result as “important — I do think it is important — extremely important, if it is true.”

“I retain some skepticism about this result, given how hard it is to make such measurement,” he added. “Especially for galaxies that are far away, small, and not very bright (I’m talking about the galaxies).”

Dr. Pandya’s team analyzed the images of galaxies in a patch of sky smaller than a full moon known as the Extended Groth Strip, which has been surveyed by many other telescopes including the Hubble telescope. The images were obtained by an international collaboration called the Cosmic Evolution Early Release Science, or CEERS, survey.

The team plans to extend its observations to other well-studied areas of the cosmos. “This will let us identify galaxies with different 3-D shapes all over the sky” and facilitate much-needed spectroscopic follow-up observations, Dr. Pandya wrote in an email.

Galaxies are the city-states of the cosmos. Within the visible universe are an estimated two trillion of them, each containing as many as a trillion stars. But the visible universe is only a fraction of what’s out there. Most of the matter in the cosmos seems to be in the form of dark matter; whatever dark matter is, it constitutes the invisible bones of the universe we see.

Astronomers now think that galaxies were seeded by random fluctuations in the density of matter and energy during the Big Bang. As space expanded, the denser areas lagged and dark matter pooled, pulling normal matter with it. This material eventually fell back together and lit up as stars and galaxies or disappeared into black holes. The Webb telescope was designed to investigate this formative and mysterious era; with a giant mirror and infrared sensors, it can see the most distant, and thus earliest, galaxies.

Dr. Pandya and his collaborators investigated the three-dimensional shapes of galaxies by statistically analyzing their two-dimensional projections on the sky. If these early galaxies were balls or disks randomly oriented in space, they should occasionally present their full faces, appearing round and circular, to telescopes.

But astronomers aren’t seeing much of that. Instead they see lots of cigars and bananas.

“They consistently look very linear,” Dr. Pandya said, “with some galaxies showing multiple bright clumps arranged like pearls on a necklace.”

Such oblong galaxies are rare today, but they make up as much as 80 percent of the galaxies in the CEERS sample, which reaches back to about 500 million years after the Big Bang.

“Their masses are such that they would be the progenitors of galaxies like the Milky Way,” Dr. Pandya said, “implying that our own galaxy may have gone through a similar cigar/surfboard morphological phase in the past.”

In the modern universe galaxies seem to come in two basic forms: featureless, roundish clouds called ellipticals, and flat, spidery discs like our Milky Way home.

Evidently the earliest newborns didn’t start out like that. The reason, astronomers suspect, is related to the properties of dark matter, but exactly which or how is unclear.

The leading theory holds that dark matter consists of clouds of exotic subatomic particles left over from the Big Bang. Ordinary matter, drawn by gravity into these clouds, would condense and light up into stars and galaxies, according to computer simulations.

In a popular variant called cold dark matter, these leftover particles would be heavy and slow compared with protons, neutrons and the other, more familiar denizens of the quantum atomic world. According to computer simulations, cold dark matter would clump easily to form the large-scale patterns astronomers see in the sky.

Identifying these slow, heavy particles would shake the world of particle physics and cosmology. But thus far experiments in labs like the Large Hadron Collider at CERN have failed to detect or produce any particles of cold dark matter. Lately, interest has shifted to other proposed forms of dark matter, including a whole gallery — a “dark sector” — of “dark” particles interacting with one another invisibly through “dark” forces.

In this mix are axions, which in theory are extremely light and act more like waves than particles — “fuzzy dark matter,” or “wavy dark matter,” in the vernacular. In computer simulations of galaxy formation, such waves can interfere with one another, producing knobby filamentary structures instead of the round shapes predicted by cold dark matter.

“Yes, the dark matter connection is tantalizing,” Dr. Pandya said, adding that the devil was in the messy details of “gastrophysics,” which describes how turbulence, hot gas and magnetic fields interact to light up stars and galaxies.

Jeremiah Ostriker, an emeritus professor of astrophysics at Princeton now affiliated with Columbia University, in recent years has turned his attention to fuzzy dark matter. In 1973, Dr. Ostriker conceived the idea of dark matter with his Princeton colleague James Peebles.

He and others have pointed out that fuzzy dark matter would leave its own signature on the sizes and shapes of baby galaxies. Because of their inherent waviness, axions would not clump as effectively as cold dark matter, so it would be hard for them to produce baby galaxies of less than one billion solar masses. Cold dark matter has no such limitation. Today’s telescopes are far from sensitive enough to observe such infants, however; a new generation of even bigger instruments may be needed to finish the job.

When Dr. Ostriker learned of Dr. Pandya’s work, he remarked that the prospects for fuzzy dark matter were looking better and better. “Keep up the good work,” he said.