The warp in the spiral disk of the Milky Way is receding under the influence of the large mass of dark matter that forms an invisible halo around our galaxy, Chinese astronomers have found.
About a third of all spiral galaxies have a distinct deformation in their disc-like structure, like a vinyl record that has been bent. It is usually the result of a number of factors; a collision with another galaxy in the past it was believed to be the main culprit in causing the distortion of the Milky Way in the first place, but further interactions with SATELLITE galaxies and intergalactics magnetic fieldas well as extensive decline clouds of gas, can also play their role. However, in the case of Milky Way at least, the main player in maintaining the warp is him dark matter halo that surrounds the disc and exerts a torque on it.
This deformation has not been fixed. Its alignment with the rest of the galaxy moves – specifically, it “preces”. Precession describes how the extent of the distortion changes with respect to the rotational axis of the galaxy, meaning that the peak, or node, of the warp precesses around the galaxy. It is a variation of the same phenomenon that causes spinning tops to wobble.
Measuring the rate of deformation precession, however, has proven challenging in the past. Previous estimates have tried to use the vertical motion of the bright but old giant STARS as a tracer to calculate the rate of precession. However, such tracers are extremely imprecise and the results based on them have suggested – contrary to theory – that the disk is undergoing prograde (in the same direction as the rotation of the rest of the galaxy) rather than retrograde (back with respect 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 tracker in the form of Cepheid variable stars to make the most accurate measure of warp precession yet, seeing as it is moving retrograde after all.
Cepheid variables are massive pulsating stars. Their pulsation period is related to how bright they are at their core and base LIGHTENING, we can calculate exactly how far they should be. This makes them excellent tracers for deformation mapping.
Huang’s team achieved their results with what they call a “moving film” method. Using data from European Space AgencyS ‘ Gaia The astrometric spacecraft, which is measuring the positions, motions and properties, including age, of more than a billion stars, Huang’s team identified a sample of 2,613 Cepheids of various ages.
“Age is the key to measuring the rate of precession of disc distortion,” the authors say in their research paper. “We obtained a moving picture of the disc distortion by mapping the three-dimensional distributions for Cepheid samples of different ages.”
Each Cepheid stores 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 in the time over the past 200 million years. At that time, by running the individual maps together, like a movie, they were able to see the advance of the deformation. They found that it is passing retrograde after all, at a rate of 2 kilometers (1.24 miles) per second for every kiloparsec (3.261 light years) to ROOM. Or, in more intuitive terms, it is moving backwards around the galaxy at a rate of 0.12 degrees every million years.
Furthermore, the movie also shows that the rate of precession decreases with distance from the galactic center, which in the long run will lead to greater distortion of the disk. Models show that this drop is the result of the dark matter halo exerting torque by being stretched or flattened.
The shape of the dark matter halo is important because it acts as a data point that theorists can feed into models that try to predict what dark matter is made of (such as WIMPs or Actions). It also provides information on formation history of the Milky Way galaxy and how it has assembled through mergers with other, smaller galaxies and gas clouds, collisions and interactions that have helped form the invisible dark matter halo.
The discovery of the rate of deformation precession is described in a paper published June 27 in Astronomy of Nature.