Faraway dwarf galaxies in the universe’s distant past — which will become modern Milky Ways — have an unexpectedly stretched-out appearance.
What did the Milky Way look like shortly after it was born? Decidedly different than it does now, according to a new study.
The study, to appear in the Astrophysical Journal, reveals that early galaxies often had surprisingly elongated shapes, some long and round like pickles, others long and flat like surfboards. And it’s these small, faraway galaxies that have become the Milky Way–like galaxies of today.
The Shape of Galaxies
“My first reaction to the paper was déjà vu!” says Lennox Cowie (University of Hawai‘i, Manoa), who conducted a seminal study on galactic shapes using Hubble data but wasn’t involved in the new work. “These observations look so like our 1995 Hubble Space Telescope observations of high-redshift galaxies.”
Studies such as Cowie’s probed galactic shapes in a universe already 2 billion years old. Yet even at this relatively mature cosmic age, some galaxies were decidedly unlike today’s full-formed galaxies, appearing elongated rather than spherical or disk-like in shape. Astronomers thought we might be viewing early disk galaxies edge-on, like frisbees seen from the side.
Now, thanks to the longer wavelengths detected by JWST, which probe farther in space and thus further back in time, images are revealing stretchd-out shapes for galaxies in a much younger universe — as young as 650 million years after the Big Bang. What’s more, it appears that the galaxies it’s finding are not edge-on disks after all: These galaxies truly are elongated.
While previous studies have examined individual galaxies, Viraj Pandya (Columbia University) and colleagues take a different approach, examining the galaxies in groups.
Pandya’s team first modeled each galaxy found in a JWST dataset known as the Cosmic Evolution Early Release Science (CEERS) survey as a three-dimensional shape called an ellipsoid. Ellipsoids can take several shapes, equivalent to the following:
- beach ball (spheroid; equally round in all directions)
- frisbee (disk; round in two directions, flat in the third direction)
- pickle (prolate; round in two directions, elongated in a third direction)
- surfboard (oval disk; elongated in one direction and flat in another)
Unless a galaxy is spheroidal, like a beach ball, its shape will depend on the angle at which we view it. And of course, we’re liable to view distant galaxies at all kinds of angles, from edge-on to face-on, thus seeing different shapes.
So Pandya’s team took an additional step: The researchers determined what 1 million ellipsoidal shapes would each look like as seen from 100,000 random viewing angles. (The process was no day at the beach!) Then they compared the distributions for different shapes to the distribution they actually observed for galaxies in a certain group.
What they found is that the farthest, smallest galaxies tend to be truly elongated — pickles and surfboards, not frisbees.
“The JWST observations are beautiful,” Cowie says. “The analysis is very careful and detailed.”
Pandya cautions that his group’s techniques can’t tell the shapes of individual galaxies, but rather the likelihood of different shapes within a group of galaxies. “Our method itself is statistical in nature,” he explains.
(For those who are curious: The researchers did not compare any galaxy shapes to bananas, even though several stories in other media outlets have referenced the fruit. The researchers used “bananas” to describe the shape their data took in their plot of the galaxies’ axis ratios, not the shape of galaxies themselves. Small, early galaxies are elongated like bananas but not necessarily bent or curved like them.)
The Meaning of Shapes
The difficulty now will be in understanding what the shapes of these galaxies are telling us about galactic evolution over cosmic time. “I do think interpretation is going to be extremely hard,” Cowie says.
First, it's worth noting that, because Pandya’s team picked star-forming galaxies to start with, they’re looking at the ancestors of modern Milky Ways and more massive lenticular (“lens-shaped”) galaxies. They are not, however, looking at the ancestors of today’s massive ellipticals, which even at those early times were already compact balls devoid of new stars.
So did the infant Milky Way start out as a pickle? That could make sense, theorists say, if its stars and gas came together in a dark matter halo that was itself elongated along a filament of the cosmic web, the network of dark matter–dominated filaments that crisscrosses the universe. Since early galaxies would have tended to line up along these filaments like cars on a highway, collisions would have occurred along a preferred direction, leading to elongated shapes.
Christopher Conselice (University of Manchester, UK), who wasn’t involved in this study, has also been studying early galaxies using JWST. “We find about a third of [early] galaxies are mergers, which is also consistent with the result of galaxies being more elongated,” he says.
Conselice also notes Pandya’s results are consistent with what his own team has found: that a galaxy’s mass is the driving factor in its evolution. “They find their lowest-mass systems are the most elongated,” he says. Lower-mass galaxies, he explains, may merge more often in the early universe, plus whatever mergers they experience can more easily change their shapes.
But Cowie thinks subtler actions may be at play. “It probably has little to do with the cosmology,” he says, “and is mostly caused by the changes in the gas inflow, the dynamics of the interstellar medium, and the structure of the star formation.”
The next aim of Pandya’s team will be to expand their set of galaxies, combining CEERS with other JWST data sets in order to grow the number of infant Milky Ways as well as their rarer, more massive siblings. With those data in hand, researchers will have their work cut out for them to understand the role of dark matter, mergers, gas flows, dust and the many other factors that play into galaxy shapes.