The first accurate 3D map of our galaxy, the Milky Way, reveals that it is warped and twisted.
Milky Way’s disc of stars becomes increasingly ‘warped’ and twisted the further away the stars are from the galaxy’s centre, astronomers from Macquarie University and the Chinese Academy of Sciences have found. The team used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy today.
From a great distance, our galaxy would look like a thin disk of stars that orbit once every few hundred million years around its central region, where hundreds of billions of stars, together with a huge mass of dark matter, provide the gravitational ‘glue’ to hold it all together.
But the pull of gravity becomes weaker far away from the Milky Way’s inner regions. In the galaxy’s far outer disk, the hydrogen atoms making up most of the Milky Way’s gas disk are no longer confined to a thin plane, but they give the disk an S-like warped appearance.
Professor Richard de Grijs, co-author of the paper and an astronomer from Macquarie University in Sydney, Australia, says; “We usually think of spiral galaxies as being quite flat, like Andromeda which you can easily see through a telescope.”
Trying to determine the real shape of our galaxy is like standing in a Sydney garden and trying to determine the shape of Australia. But, for the past 50 years, there have been indications that the hydrogen clouds in the Milky Way are warped. The new map shows that the warped Milky Way disc also contains young stars. It confirms that the warped spiral pattern is caused by torque from the spinning of the Milky Way’s massive inner disc of stars.
Xiaodian Chen, lead author, from the Chinese Academy of Sciences in Beijing, says: “It is notoriously difficult to determine distances from the Sun to parts of the Milky Way’s outer gas disc without having a clear idea of what that disc actually looks like.”
“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% can be determined.” That database allowed the team to develop the first accurate three-dimensional picture of our Milky Way out to its far outer regions.
Classical Cepheids are young stars that are some four to 20 times as massive as our Sun and up to 100,000 times as bright. Such high stellar masses imply that they live fast and die young, burning through their nuclear fuel very quickly, sometimes in only a few million years. They show day- to month-long pulsations, which are observed as changes in their brightness. Combined with a Cepheid’s observed brightness, its pulsation period can be used to obtain a highly reliable distance.
Prof. Richard de Grijs from Macquarie University in Sydney, Australia, and senior co-author of the paper, adds: “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.”
“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.”
This reminded the team of earlier observations of a dozen other galaxies which also showed such progressively twisted spiral patterns. Astronomers have observed a dozen other galaxies which showed similar progressively twisted spiral patterns in their outer regions. So our Milky Way’s twists may be rare but they are not unique.
Dr Liu Chao, senior researcher and co-author of the paper, says: “Combining our results with those other observations, we concluded that the Milky Way’s warped spiral pattern is most likely caused by ‘torques’ — or rotational forcing — by the massive inner disk.”
Licai Deng, a senior researcher at the Chinese Academy of Sciences and co-author on the paper, adds: “This research provides a crucial updated map for studies of our galaxy’s stellar motions and the origins of the Milky Way’s disk.”
“This new morphology provides a crucial updated map for studies of our galaxy’s stellar motions and the origins of the Milky Way’s disk,” says Dr Deng Licai, senior researcher at NAOC and co-author of the paper.
Original research: http://dx.doi.org/10.1038/s41550-018-0686-7