The ripples of the Milky Way’s spiral disk are moving backward under the influence of a huge mass of dark matter that forms an invisible halo around our galaxy, Chinese astronomers have found.
About one third of all spiral galaxy they have a distinct deformation of their disk-like structure, like a vinyl record that has been bent. It is usually the result of various factors; collision with another galaxy in the past it was believed to be the main culprit in causing the Milky Way to warp in the first place, but other interactions with satellite galaxy and intergalactic magnetic fieldas well as the decline of extensive clouds gas can also play a role. However, in case Milky Way at least the main player in maintaining the warp is dark matter the halo that surrounds the disc and exerts a torque on it.
This deformation is not fixed. Its alignment with the rest of the galaxy is moving—specifically, “precessing.” Precession describes how the alignment of the warp changes with respect to the axis of rotation galaxy, meaning that the peak or node of the warp processes passes around the galaxy. It is a variation of the same phenomenon that causes spinning wheels to wobble.
However, measuring the rate of warp precession has proven challenging in the past. Previous estimates attempted to use the vertical motion of a bright but old giant stars as trackers to calculate the precession rate. However, these indicators are notoriously imprecise, and the results based on them suggested – contrary to theory – that the disc is pre-progressive (in the same direction as the rest of the galaxy’s rotation) and not retrograde (back to the galaxy). galaxy) as expected.
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Now, astronomers led by Yang Huang of the Chinese Academy of Sciences have used a different, more precise tracker in the form Cepheid variable stars to make the most accurate measurement of warp precession to date and finally find that it is moving retrograde.
Cepheid variables are pulsating massive stars. Their pulsation period is related to how bright they are internally and based on their luminosity, we can calculate exactly how far it must be. This makes them great tracers for warp mapping.
Huang’s team achieved their results using a method they call a “movie frame.” Using data from European Space Agency‘with Gaia 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 of varying ages.
“Age is the key to measuring the rate of precession of disc deformation,” the authors say in their research paper. “We obtained a movie of disk deformation by mapping three-dimensional distributions for Cepheid samples of different ages.”
Each Cepheid retains information about its position in the warp when it was born, so by grouping the Cepheids into different age groups and mapping them, Huang’s team was able to show the shape and position of the warp at different points time over the past 200 million years. Then when they ran the individual maps together, like a movie, they were able to see the progress of the warp. They found that it is processed retrograde after all, at a rate of 2 kilometers (1.24 miles) per second for every kiloparsec (3,261 light years) of space. Or, in more intuitive units, it travels back around the galaxy at a rate of 0.12 degrees per million years.
What’s more, the movie also shows that the precession rate decreases with distance from the galactic center, which will lead to more deformation of the disc in the long run. The models suggest that this dip is the result of a dark matter halo developing a torque, having a flattened or flattened shape.
The shape of the dark matter halo is important because it acts as a data point that theorists can plug into models that try to predict what dark matter is made of (such as WIMPs or axions). It also provides clues about formation history the Milky Way galaxy and how it was assembled through mergers with other, smaller galaxies and clouds of gas, collisions and interactions that helped form the invisible dark matter halo.
The discovery of the warp precession rate is described in a paper published on June 27 Astronomy of nature.