The Milky Way Is More Twisted Than Previously Thought

Artist's impression of the warped and twisted Milky Way disk. CHEN Xiaodian

It has been difficult to paint an accurate picture of what our galaxy looks like. Being inside of it puts us at a disadvantage and we are constantly discovering new characteristics of our “little” corner of the universe. Now, research published in Nature Astronomy adds another one, the disk of the Milky Way becomes more warped and twisted the further you get from the core.

The Milky Way is a fairly typical spiral galaxy. It has a bar-shaped central region which is quite thick, but most of its stars are organized in a thin disk of hydrogen atoms and stars that stretches 50,000 light-years from the core. The disk was thought to be mostly thin and flat, but the new study argues that it is anything but.

A team from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) tracked stars in the outer disk and was able to measure their position precisely enough to construct a fairly accurate 3D picture of the disk. It seems the disk at that distance has a curious S-shape.

"It is notoriously difficult to determine distances from the Sun to parts of the Milky Way's outer gas disk without having a clear idea of what that disk actually looks like," lead author Dr Chen Xiaodian said in a statement. "However, we recently published a new catalogue of well-behaved variable stars known as classical Cepheids, for which distances as accurate as 3 to 5 percent can be determined.”

Cepheids are a special class of star whose day-to-month-long pulsation depends on their true brightness. When that is combined with measurements of their observed brightness (the further away they are, the dimmer they appear), researchers can use them to get a pretty good estimate of their distance. This relation, devised by American astronomer Henrietta Swan Leavitt, established the first standard candle and led to the discovery of the expansion of the universe.

"Somewhat to our surprise, we found that in 3D our collection of 1339 Cepheid stars and the Milky Way's gas disk follow each other closely. This offers new insights into the formation of our home galaxy," senior co-author Professor Richard de Grijs from Macquarie University in Sydney, Australia, explained. "Perhaps more importantly, in the Milky Way's outer regions, we found that the S-like stellar disk is warped in a progressively twisted spiral pattern."

The team believes that the twist in the outer disk is due to rotational forces (aka torques) from the massive inner disk. Such warped disks have been observed in other galaxies as well. More observations, such as those currently being conducted by ESA's Gaia mission, might give even us more information about the Milky Way.

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