Astronomers have been spotting strange banana-shaped galaxies and the evidence seems to indicate that filaments of dark matter make them take this shape.
A surprising number of distant “banana-shaped” galaxies spotted by the James Webb Space Telescope (JWST) could be tracing a vast web of dark matter from the universe’s first moments, and may even be a sign of strange kinds of dark matter. Working out which types could help us understand how galaxies across the universe formed.
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| Vaguely banana-shaped galaxies detected by JWST (top row) and simulations of how galaxies would look if formed under the influence of wave dark matter (row 2), warm dark matter (row 3) and cold dark matter (row 4) Alvaro Pozo et al. (2024) |
Astronomers assume that most galaxies start out as fairly symmetrical discs before evolving into more complex swirls like our own Milky Way. However, in the 1990s the Hubble Space Telescope started spotting elongated, or what astronomers call prolate, galaxies in the early universe that didn’t fit with this picture.
Many researchers thought this might be a result of Hubble’s view of the scene – it was only seeing the brightest galaxies, which could either be prolate or disc galaxies seen from the side, and missing the dimmer disc galaxies that are orientated face-on to Earth.
People also suggested that the gravity from massive galaxies between the distant galaxies and us might be distorting the light coming from the distant ones to make them appear more elongated, an effect called gravitational lensing. But some astronomers suggested that dark matter, an invisible particle or force that exerts a powerful gravitational pull, might be responsible.
Last year, Viraj Pandya at Columbia University in New York and his colleagues started spotting many more of these elongated galaxies with JWST, which they called “banana galaxies”, in the early universe between 8 billion and 13 billion years ago. JWST is much more sensitive than Hubble and should be able to see the face-on disc galaxies if they exist. But the normal disc galaxies seemed to still be missing.
“At a certain point, you have to throw your hands up and say, ‘Well, okay, maybe this is really a fundamental puzzle’,” says Pandya.
Now, Pandya and his colleagues have calculated that gravitational lensing would not produce enough of an effect to elongate these galaxies so much, which strengthens the picture that these aren’t disc galaxies seen from an odd angle. “If that is the correct solution, that just throws our galaxy formation theories into disarray,” says Pandya.
The researchers also analysed whether the galaxies were pointing in random directions or whether they lined up in any sort of pattern. To do this, they took different pictures from JWST of the same patch of sky and measured the average orientation across all of them. If the galaxies were randomly orientated, the average orientation of all of them should be zero, but instead they found that the galaxies were statistically aligned.
This could be because some dark matter we don’t know about is sitting between us and the galaxies, and is aligning them, but we don’t know of any large galaxy clusters, which themselves contain dark matter, that might be responsible in these regions of the sky, says Pandya.
Another explanation could be that these galaxies are actually forming along filaments of dark matter in the early universe, revealing the complex structure of the early universe that is otherwise invisible (see “The dark matter web”, below).
“These elongated galaxies could be like little light bulbs and they’re tracing out the filaments,” says Pandya. However, we will need to calculate more accurate distances to these galaxies before we can distinguish between these two scenarios, he says.
The dark matter filaments that may be responsible for these elongated galaxies could also be exotic in nature. In another study, Álvaro Pozo at the University of the Basque Country in Spain and his colleagues have run simulations of the early universe to see what shape the galaxies would form, and at which times, according to different models of dark matter.
They tested what would have happened with cold dark matter, the slow-moving and inert dark matter particle that forms the standard cosmological model; warm dark matter, where particles are faster moving; and wave dark matter, where particles are smeared out across space like ripples on a lake.
Pozo and his colleagues found that cold dark matter only produced elongated galaxies at much earlier times in the universe’s history than those identified by Pandya. But the warm and wave dark matter simulations produced strikingly similar galaxy patterns of the same age as those spotted by Pandya.
If we can work out what form dark matter takes, then we can understand how and when galaxies and the large-scale structure of the universe form, says Claudia Maraston at the University of Portsmouth, UK. In cold dark matter, for instance, structures form from the bottom up. Galaxies group together, and then groups themselves form clusters, until the universe’s structure is determined. But in warm dark matter, the large-scale structure determines the formation and behaviour of smaller objects, like galaxies.
Finding possible evidence of dark matter filaments and verifying that these galaxies really do have these elongated shapes is very exciting, says Maraston. But we have only seen a small number of prolate galaxies with JWST so far and will need to observe many more with both JWST and future galaxy surveys like the European Space Agency’s Euclid telescope before we can be sure that there aren’t still observational biases, she says.
The galaxy simulations from Pozo and his team produce convincingly similar images to what physicists observe, but the simulation only runs to relatively early in the universe’s history, says Maraston. To convince astronomers that warm or wave dark matter is correct, the simulations will also have to reproduce what we see in our most recent universe, she says.
Reference:
arXiv DOI: 10.48550/arXiv.2407.17552, DOI: 10.48550/arXiv.2407.16339
The dark matter web
We know that galaxies and dark matter go hand in hand. While we can’t see dark matter using light, we can observe it through its gravity, which affects the structure and movement of galaxies across our universe. Astronomers have mapped out this dark matter by looking at nearby galaxies and found a vast web of filaments connecting different clumpy regions.
Charting these filaments in the early, faraway universe is much more difficult because we can’t accurately measure how far away the galaxies are. Our current best method is to use distant and bright black holes called quasars as flashlights to illuminate slightly closer regions of the universe, but we don’t know if these regions are representative of the entire web.
Tracing the alignments of banana galaxies (see main article) might provide an alternative way to chart this dark matter, which is crucial for our understanding of cosmology. By mapping out basic properties about these filaments, like their number, length and thickness, we can build better cosmological models, says Viraj Pandya at Columbia University in New York, and find out which theories best match up to reality.
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