So it’s confession time: I’ve been mendacity to you.
I’ve mentioned on many events that our Milky Means galaxy has a flat disk (like on this column or this one). But it surely’s not likely flat—not even for an affordable definition of the time period.
Now, in my protection, I wasn’t mendacity per se; I used to be simplifying. That’s a superbly acceptable and even advantageous factor to do in science. When you could have some complicated factor that you just’re attempting to know or clarify, it helps to make it so simple as potential in order that the maths and physics are simpler to crack. It’s like assuming, at first, that Earth is an ideal sphere or that the solar comprises all of the mass within the photo voltaic system. As soon as you’re employed out the essential equations that describe your simplified mannequin, you possibly can progressively add complexity again in—however in a means that makes the issue tractable.
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And to be truthful, while you take a look at the glow of the Milky Means from a darkish web site, it does look flat—flat-ish. And we a lot of related galaxies and their disks additionally seem flat.
However a variety of them, perhaps even most of them, aren’t. They’re wiggly and wavy and flexible. Our galaxy is amongst this warped group.
First, a fast overview: the Milky Means is classed as a disk galaxy, with a broad round assortment of stars, fuel and dirt about 120,000 light-years throughout. It’s a couple of thousand light-years thick, so “flat” is not less than a good adjective to make use of for it. Within the heart is a central bulge of stars, and the entire thing is surrounded by an enormous halo of stars and darkish matter about one million light-years large.
That final bit is vital. Cling on a minute, and I’ll clarify why.
We’ve identified for a while that, out towards its edges, the disk of the Milky Means is warped, flared up on one facet and down on the opposite, fairly just like the brim of a fedora. Analysis revealed within the journal Science in 2019, nonetheless, refined this concept significantly. The group of astronomers behind that paper used information from Gaia, a now retired European House Company mission that mapped the positions, motions and distances of greater than a billion stars. They particularly checked out Gaia’s information for some 2,400 Cepheid variables—particular sorts of stars that pulsate, altering their brightness. The time it takes for a Cepheid’s brightness to alter is expounded to its luminosity, the quantity of vitality it provides off. By evaluating a Cepheid’s intrinsic luminosity with how brilliant the star seems in our sky, its distance can then be calculated.
By mapping so many Cepheids within the galactic aircraft, the scientists had been in a position to hint the general form of the Milky Means’s disk, and the warping actually stands out. Our galaxy appears a bit like a vinyl LP that’s been out within the solar too lengthy (youngsters, ask your grandparents).
What causes such a warp? It’s potential {that a} collision with a smaller galaxy may gravitationally have an effect on the celebs within the disk, a bit like ripples in a pond after a rock is tossed in. However a group of astronomers who revealed their analysis in Nature Astronomy in 2023 had a really completely different concept for what’s been tugging on our galaxy’s brim: darkish matter.
As I discussed above, the galaxy is embedded in a halo of stars and darkish matter. A couple of yr earlier than their paper, a number of the members of this group, together with different astronomers, discovered that the stellar halo was not spherical, as beforehand assumed, however as a substitute elongated and squished just a little, a bit like a barely flattened American soccer. It was additionally tilted with respect to the aircraft of the galaxy.
They posited that the extra diffuse and nigh-invisible darkish matter halo may need the identical form because the stellar halo as effectively. By modeling the results of the rather more huge darkish matter halo if it had been structured and oriented in an analogous means, they discovered that this naturally created a gravitational subject that tugged on the disk, which might clarify not simply the form and dimension of the warp but additionally its orientation within the disk. Whereas this isn’t essentially case closed, they make a reasonably stable argument.
However that’s not the one means our galaxy’s disk is off-kilter. New analysis reveals it’s additionally corrugated.
Simply this yr one other group of scientists used Gaia information to take a look at 17,000 younger stars—which are likely to type proper in the course of the galaxy’s disk—and three,400 Cepheid variables in a area of the Milky Means tens of hundreds of light-years throughout. What they discovered is that, in the principle disk and effectively into the warped outer elements, there may be an up-and-down wave, a construction very like the corrugation in thick cardboard.
It’s just like the “wave” followers do in at sports activities video games, the place they arise and sit again down in a wave that strikes across the stadium. In our case, the celebs within the galaxy transfer up and down relative to the aircraft of the disk. As a result of so lots of the stars they measured are younger, the scientists suppose the fuel within the galaxy—which types stars—strikes up and down as effectively. So no matter that is, it’s intrinsic to the construction of the disk.
The reason for this wave isn’t identified, although the almost certainly wrongdoer is, this time, a collision with a smaller galaxy. One potential, even doubtless, responsible get together is the Sagittarius Dwarf Spheroidal galaxy, a small object with a tiny fraction of the Milky Means’s mass. It orbits our galaxy in an almost vertical loop, diving by way of the disk because it goes. In 2018 astronomers revealed a paper in Nature the place they discovered—once more, utilizing Gaia information—wavelike motions in six million stars inside roughly 10,000 light-years of the solar, just like the waves discovered within the outer disk. They recommend that the Sagittarius galaxy could have created these constructions the final time it handed by way of the disk, a number of hundred million years in the past.
The solar additionally shows this movement; cautious measurements present it has a vertical velocity because it orbits the galactic heart. This implies our photo voltaic system bobs up and down. And occasionally, it strikes by way of the disk, reaches a long way from it, after which the gravity of the disk pulls it again, and the cycle begins once more. This component of our star’s movement could also be part of that larger wave.
In order you possibly can see, I wasn’t mendacity earlier than about our flat disk. I used to be merely eliding over particulars that aren’t obligatory in a dialogue of the general construction of the galaxy. But it’s definitely value taking a look at these additional results—they inform us concerning the historical past of our Milky Means and might even reveal how the solar performs its half as effectively.
