Astronomers measure 'warp speed' of Milky Way galaxy

The warp in the Milky Way's spiral disk is precessing backward under the iNFLuence of the enormous mass of dark matter that forms an invisible halo around our galaxy, Chinese astronomers have discovered.

About one-third of all spiral galaxies have a distinct warp to their disk-shaped structure, like a vinyl record that has been bent. It's usually the result of a variety of factors; a collision with another galaxy in the past is believed to be the primary culprit in causing the Milky Way's warp in the first place, but further interactions with satellite galaxies and the intergalactic magnetic field, as well as the infall of vast clouds of gas, can also play their part. However, in the case of the Milky Way at least, the major player in maintaining the warp is the dark matter halo that surrounds the disk and exerts a torque on it.

This warp isn't fixed. Its alignment with the rest of the galaxy moves — specifically, it "precesses." Precession describes how the alignment of the warp changes with respect to the rotational axis of the galaxy, meaning that the peak, or node, of the warp precesses around the galaxy. It's a variation of the same phenomenon that causes spinning tops to wobble. 

Measuring the warp's rate of precession, however, has proven challenging in the past. Previous estimates have attempted to use the vertical motion of bright, but old, giant stars as tracers to calculate the rate of precession. However, such tracers are notoriously imprecise, and results based on them had suggested — counter to theory — that the disk is precessing prograde (in the same direction as the rotation of the rest of the galaxy) and not retrograde (backward with respect to the galaxy), as had been expected.

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Now, astronomers led by Yang Huang of the Chinese Academy of Sciences have used another, more accurate tracer in the form of Cepheid variable stars to make the most accurate measure of the warp's precession yet, finding it to be moving retrograde after all.

Cepheid variables are pulsating massive stars. Their period of pulsation is linked to how intrinsically bright they are, and based on their luminosity, we can calculate exactly how far away they must be. This makes them great tracers for mapping the warp.