So, it's time to confess: I lied to you.
I have said many times that our Milky Way galaxy has a flat disk (like in this column or This one). But it's not entirely flat—even by a reasonable definition of the term.
Now, in my defense, I wasn't lying per se; I was simplification. This is quite acceptable and even beneficial in science. When you have some complex thing that you are trying to understand or explain, it is useful to make it as simple as possible so that it is easier to understand the mathematics and physics. This is like assuming that the Earth is a perfect sphere or that the Sun contains all the mass of the solar system. Once you have developed the basic equations that describe your simplified model, you can gradually add complexity back in a way that makes the problem tractable.
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And frankly, when you look at the Milky Way's glow from a dark place, it does look flat, flat. And many similar galaxies and their disks also appear flat.
But many of them, maybe even most of them, are not. They are twisty, wavy and flexible. Our galaxy is among this distorted group.
First, a quick overview: The Milky Way is classified as a disk galaxy with a wide circular collection of stars, gas and dust about 120,000 light-years in diameter. It's several thousand light years thick, so “flat” is at least an appropriate adjective for it. At the center is a central bulge of stars, and the whole thing is surrounded by a huge halo of stars and dark matter about a million light years across.
This last bit is important. Wait a minute and I'll explain why.
We have known for a long time that closer to the edges the disk of the Milky Way is curved, expanding on one side and falling on the other, like the brim of a hat. The study was published in the journal Science however in 2019 significantly improved this idea. The team of astronomers who wrote this article used data from Gaia, now retired A European Space Agency mission that mapped positions, movements and distances over billion stars. They specifically looked at Gaia data for about 2,400 Cepheid variables—special types of stars that pulsate, changing their brightness. The time it takes for a Cepheid to change in brightness is related to its luminosity, the amount of energy it emits. By comparing the internal luminosity of a Cepheid with the brightness of a star in our sky, we can calculate its distance.
By mapping so many Cepheids in the galactic plane, scientists were able to trace the overall shape of the Milky Way's disk, and its deformation really stands out. Our galaxy is a little like a vinyl record that's been in the sun too long (kids, ask your grandparents).
What causes this imbalance? It's possible that a collision with a smaller galaxy could have a gravitational effect on the stars in the disk, something like ripples in a pond after a thrown stone. But a team of astronomers publishing their research in the journal Nature Astronomy in 2023 had a very different idea. what pulls at the edges of our galaxy: dark matter.
As I mentioned above, the galaxy is surrounded by a halo of stars and dark matter. About a year before their article, some members of this group, along with other astronomers discovered that the stellar halo is not spherical.as previously assumed, but instead became slightly longer and flattened, somewhat reminiscent of a slightly flattened American football. It was also tilted relative to the plane of the galaxy.
They suggested that a more diffuse and nearly invisible dark matter halo could have the same shape as a stellar halo. By simulating the effects of a much more massive dark matter halo if it were structured and oriented in a similar way, they found that this naturally creates a gravitational field that pulls on the disk, which explains not only the shape and size of the warp, but also its orientation within the disk. While the case isn't necessarily closed, they make a pretty strong case.
But this is not the only case where the disk of our galaxy is out of order. New research shows that it is also corrugated.
Just this year, another team of scientists used Gaia data to study 17,000 young stars, which tend to form right in the middle of a galaxy's disk, and 3,400 Cepheid variables in a region of the Milky Way tens of thousands of light-years across. What did they find is that there is an up and down wave in the main disk and in the deformed outer parts, the structure is very similar to the corrugation of thick cardboard.
This is similar to the “wave” that fans do at sports games, where they stand up and sit down in a wave that moves around the stadium. In our case, the stars in the galaxy move up and down relative to the plane of the disk. Because many of the stars they measured are young, scientists believe that the gas in the galaxy that forms the stars is also moving up and down. Whatever it is, it is inherent in the structure of the disk.
The cause of this wave is unknown, although the most likely culprit this time is a collision with a smaller galaxy. One of the potential and even probable culprits is Sagittarius Dwarf Spheroidal Galaxya small object with a tiny fraction of the mass of the Milky Way. It orbits our galaxy in a nearly vertical loop, diving across the disk. In 2018 astronomers published a paper in Nature where they found – again using Gaia data –undulating motions in six million stars about 10,000 light-years from the Sun, similar to the waves found in the outer disk. They suggest that the Sagittarius galaxy may have created these structures during its last passage through the disk, several hundred million years ago.
The sun also exhibits this movement; careful measurements show that it has a vertical speed as it revolves around the center of the galaxy. This means that our solar system is wobbling up and down. And from time to time it moves through the disk, reaches some distance from it, and then the gravity of the disk pulls it back, and the cycle begins again. This element of our star's motion may be part of this larger wave.
So, as you can see, I didn't lie about our flat disk. I simply left out details that are not needed when discussing the overall structure of the galaxy. However, these additional effects are certainly worth paying attention to – they tell us about the history of our Milky Way and may even show what role the Sun plays.
