Astronomers measure 'warp speed' of Milky Way galaxy

Astronomers have made a significant breakthrough in measuring the "warp speed" of the Milky Way galaxy. Chinese astronomers, studying the spiral disk of the Milky Way, have discovered that the warp is precessing backward due to the influence of a massive halo of dark matter surrounding the galaxy. This dark matter halo exerts a torque on the spiral disk, maintaining its distinctive warp.

Around one-third of spiral galaxies, including the Milky Way, exhibit this warped disk structure, akin to a bent vinyl record. The primary cause of the warp in our galaxy is believed to be a collision with another galaxy in the distant past. However, other factors such as interactions with satellite galaxies, the intergalactic magnetic field, and the interstellar infall of gas can also contribute. Nonetheless, the dark matter halo is shown to play a major role in sustaining the warp in the Milky Way.

Notably, the alignment of the warp is not fixed but undergoes precession, meaning that its node or peak steadily moves in relation to the galaxy's rotational axis. This phenomena resembles the wobbling of a spinning top. Past efforts to measure the warp's rate of precession faced challenges. Previous estimates utilized the vertical motion of old giant stars as tracers, but their imprecise nature resulted in conflicting conclusions.

Now, a team of astronomers led by Yang Huang from the Chinese Academy of Sciences, employing more accurate tracers called Cepheid variable stars, have made the most precise measurement of the warp's precession to date. Cepheid variables are pulsating massive stars whose brightness is linked to their pulsation period, enabling the calculation of their distance from Earth. This characteristic makes them excellent tracers for mapping the warp.

Huang's team developed a novel approach termed the "motion picture" method. Utilizing data from the European Space Agency's Gaia astrometric spacecraft, which collects data on the positions, motions, and properties of billions of stars, the team identified 2,613 Cepheids across various ages. Since the age is crucial in measuring the precession rate of the disk warp, the team used the distribution of these Cepheids, grouping them into different age ranges and mapping their positions within the warp. By compiling these individual maps, they created a motion picture that revealed the warp's precession.

The team's findings indicate that the warp is indeed precessing retrograde, moving backward around the galaxy at a rate of 2 kilometers (1.24 miles) per second for every kiloparsec (3,261 light-years) of space. This corresponds to approximately 0.12 degrees of precession every million years. Additionally, the motion picture unveiled a decrease in the precession rate with distance from the galactic center, suggesting that the disk will experience greater warping in the long run. This decrease is believed to be caused by the oblate shape of the dark matter halo exerting torque on the warp.

Understanding the shape of the dark matter halo is pivotal for scientists as it provides insights into the composition of dark matter and the formation history of the Milky Way. Researchers can incorporate this information into models aimed at unraveling the nature of dark matter and comprehending the assembly of our galaxy through interactions with smaller galaxies, gas clouds, and collisions. The breakthrough regarding the precession rate of the Milky Way's warp has been published in a research paper in Nature Astronomy on June 27.