“Is the Milky Way Spinning Like a Top?”

The Milky Way Galaxy isn’t the flat spiral shape you probably imagine, according to new research by  astrophysicists at the Instituto de Astrofísica de Canarias, “rather it’s warped like the brim of a fedora hat with one side pointed up and the opposing side pointed down.” Their findings call into question a claim that the warp is precessing, or wobbling, at a significant rate over time.

Moves in a Cycle Every 600-700 Million Years 

In 2020 scientists published that the Milky Way’s warp was precessing — or wobbling like a decaying spinning top– at a rate that greatly exceeded theoretical predictions. Researchers found that stars in the warp followed a 600-700 million year cycles, a timespan that is three times the orbital period of the Sun  traveling around the center of the Galaxy.Home 

Our Warped Galaxy

The Milky Way is a spiral galaxy, which means that it is composed, among other components, of a diskc of stars, gas and dust, in which the spiral arms are contained. At first, it was thought that the diskc was completely flat, but for some decades now it is known that the outermost part of the diskc is distorted into what is called a “warp”: in one direction it is twisted upwards, and in the opposite direction downwards. The stars, the gas, and the dust are all warped, and so are not in the same plane as the extended inner part of the disc, and an axis perpendicular to the planes of the warp defines their rotation.

The Real Milky Way –“Our Warped, Twisted, Wobbly Galaxy”

Precession is not a phenomenon which occurs only in galaxies, it also happens to our planet. As well as its annual revolution around the Sun, and its rotation period of 24 hours, the axis of the Earth precesses, which implies that the celestial pole is not always close to the present pole star, but that (as an example) 14,000 years ago it was close to the star Vega.

Image of the precessing warp of the Milky Way disc is shown below. (Gabriel Pérez Díaz, SMM, IAC).

Younger Stars in Gaia Mission Data Have a Much Larger Warp

Now, the new study by researchers Žofia Chrobáková and Martín López Corredoira has taken into account the variation of the amplitude of the warp with the ages of stars examined. The study concludes that, using the warp of the old stars whose velocities have been measured, it is possible that the precession can disappear, or at least become slower than what is presently believed. To arrive at this result the researchers have used data from the Gaia Mission of the European Space Agency (ESA), analysing the positions and velocities of hundreds of millions of stars in the outer disc.

Ghosts of the Milky Way –“Have Our Galaxy Ringing Like a Bell”

“In previous studies it had not been noticed,” explains Žofia Chrobáková, a predoctoral researcher at the IAC and the first author of the article, “that the stars which are a few tens of millions of years old, such as the Cepheids, have a much larger warp than that of the stars visible with the Gaia mission, which are billions of years old.”

“There are some hypotheses to explain why the younger stars have a warp larger than the older stars,” replied Martín López Corredoira to an email from The Daily Galaxy: ” the warp might be produced by a torque that affects mainly to the gas (and the young stars embedded on it), whereas the old stars trend to recover the flat shape due to self-gravity of the disk. This torque cannot be gravitational (which affects equally young and old  stellar populations) and might be produced for instance by some kind of friction in the interaction with the accretion of the intergalactic medium.”

“This does not necessarily mean that the warp does not precess at all, it could do so, but much more slowly, and we are probably unable to measure this motion until we obtain better data,” concludes Corredoira, an IAC researcher and co-author of the paper..

Source: Ž. Chrobáková et al, A Case against a Significant Detection of Precession in the Galactic Warp, The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/abf356

Image at the top of the page is an artist’s impression of the warped and twisted Milky Way disk, by Chen Xiaodian, based on the first accurate 3D map of our galaxy revealing its true shape. Astronomers from Macquarie University and the Chinese Academy of Sciences used 1339 ‘standard’ stars to map the real shape of our home galaxy.

Jackie Faherty, astrophysicist, Senior Scientist with AMNH via Instituto de Astrofísica de Canarias. 

Jackie Faherty

Editor, Jackie Faherty, astrophysicist, Senior Scientist with AMNH. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterising planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighbourhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognises scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.