Chinese astronomers have discovered that the warp in the Milky Way’s spiral disk is moving backwards under the influence of the enormous mass of dark matter that forms an invisible halo around our galaxy.
About a third of all spiral galaxies have a distinct curvature in their disc-shaped structure, like a vinyl record that has been bent. It is usually the result of several factors; a collision with another galaxy has historically been thought to be the main culprit for the curvature of the Milky Way, but further interactions with satellite galaxies and the intergalactic magnetic fieldas well as the invasion of huge clouds of gas, can also play their role. However, in the case of the Milky Way in any case, the most important player in maintaining the warp is the dark matter halo that surrounds the disk and exerts a torque on it.
This warp is not fixed. Its alignment with the rest of the galaxy moves — specifically, it “precesses.” Precession describes how the warp’s alignment changes relative to the galaxy’s rotational axis. universewhich means that the peak, or knot, of the warp is precessing around the galaxy. It is a variation of the same phenomenon that causes spinning tops to wobble.
Measuring the warp precession rate has proven challenging in the past, however. Previous estimates have attempted to measure the vertical motion of bright but old, giant stars as tracers to calculate the precession rate. However, such tracers are notoriously inaccurate, and results based on them suggested—contrary to theory—that the disk is precessing progradely (in the same direction as the rotation of the rest of the galaxy) rather than retrogradely (backwards relative to the galaxy) as expected.
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Now, astronomers led by Yang Huang of the Chinese Academy of Sciences have used another, more precise tracer in the form of Cepheid variable stars to measure the precession of the curvature as accurately as possible, and to discover that it ultimately moves retrograde after all.
Cepheids are pulsating, massive stars. Their pulsation period is tied to how intrinsically bright they are, and based on their Brightnesswe can calculate exactly how far away they need to be. This makes them great tracers for mapping the warp.
Huang’s team achieved their results using what they call the “motion picture” method. Using data from the European Space Agency‘S Gaia Using an astrometric spacecraft that measures the positions, motions and properties, including ages, of more than a billion stars, Huang’s team identified a sample of 2,613 Cepheids with varying ages.
“Age is key to measuring the precession rate of disk deformation,” the authors say in their research paper. “We obtained a moving picture of disk deformation by mapping the three-dimensional distributions for Cepheid samples of different ages.”
Each Cepheid retains information about its position in the warp when it was born. By grouping the Cepheids into different age groups and mapping them, Huang’s team was able to map the shape and position of the warp at different points in time. time over the past 200 million years. By then piecing the individual maps together, like a movie, they could see the warp precessing. They found that it is ultimately precessing retrograde, at a rate of 2 kilometers (1.24 miles) per second for every kiloparsec (3.261 light years) by roomOr, to put it more intuitively, it moves backward around the galaxy at a rate of 0.12 degrees every million years.
Furthermore, the movie also shows that the precession rate decreases with distance from the galactic center, which will lead to more warping of the disk in the long term. Models indicate that this decrease is the result of the dark matter halo exerting the torque, which is flattened in shape.
The shape of the dark matter halo is important because it serves as a data point that theorists can use in models that try to predict what dark matter is made of (such as WIMPs or axions). It also gives clues about the formation history of the Milky Way galaxy and how it formed through mergers with other, smaller galaxies and gas clouds, collisions and interactions that contributed to the formation of the invisible halo of dark matter.
The discovery of the precession rate of curvature is described in a paper published June 27 in Nature Astronomy.