How do spiral galaxies get 'feathers'? All it takes is a little gravity

Galaxies like the Milky Way boast more than spirals; they also have feathers, where clumps of new stars are born. Astronomers have been stumped by how these complex structures form, but now a research team says there may be a simple way: All it takes is a bit of gravity.

For over a century, astronomers have marveled at the beauty and complexity of spiral galaxies. The largest "grand design" spirals can host a dozen or more individual arms. But recently, detailed, high-resolution observations with the Hubble Space Telescope and the James Webb Space Telescope have revealed that there is even more to the story.

A single arm can wind for tens of thousands of light-years around a galaxy. But astronomers have found that decorating those arms are a multitude of small-scale features known as feathers.

These feathers are much smaller than arms, reaching no more than a few thousand light-years in length. But they are exceptionally dense — much denser than the larger arms. They are also home to intense star formation. Within the feathers, astronomers have spotted young star clusters and giant clumps of neutral hydrogen. Indeed, much — and perhaps most — of star formation within a galaxy takes place inside these feathers.

At first, astronomers thought feathers were a feature of only the largest grand-design spirals. But the evidence is mounting that almost all spiral galaxies, including the Milky Way, host feathers.

So how does a galaxy get these feathers? Over the years, debates have sprung up as astronomers have proposed various mechanisms to explain how feathers form. Feathers are relatively small, at least on galactic scales. They're also numerous. So the thinking is that there must be some complex process that builds them. For example, perhaps supernova explosions shape the feathers from spiral arms, carving into the gas like a sculpture. Or perhaps weak-but-gigantic magnetic fields twist up on themselves to create the filamentary structures.

But in new work accepted for publication in the journal Astronomy & Astrophysics, astronomers propose something much, much simpler: gravity. Just gravity.

To test this hypothesis, the team created an exceptionally simple simulated galaxy. This mock galaxy did not contain any stars or clumps of matter, nor did it involve supernovas or magnetic fields. It was just a simple, rotating disk of gas. Then, they let this disk of gas evolve under its own gravity to see if feathers naturally appeared.

And they did. In just a short time, the simulated disks of gas fragmented into a series of nested filaments that resembled the feathers from observations — no complex physics necessary. That's because the disk of gas was unstable, with any tiny clumps prone to collapsing on themselves. That natural inclination, combined with the rotation of the gas, produced small, elongated structures — the feathers.

Then, the researchers compared the feathers produced in simulated galaxies with actual feathers in observations. They found generally broad agreement in size, shape and density.

The simulated galaxies didn't look anything like real galaxies. But that's the point: The researchers wanted to examine whether simple physics — just gravity by itself — could produce feathers. It seems that this is the case, but the real test will be to (ironically) introduce more complex physics into the simulations. This is because things like supernovas and magnetic fields do influence how spiral galaxies evolve, and they may break apart these gravitationally powered feathers — which means we would be back to square one.

Still, it's a promising hypothesis and shows that nature is perfectly capable of using simple physics to create complex structures, even at galactic scales.