01 August 2019 22:47
The most detailed three-dimensional map yet of the Milky Way has been revealed, showing that our galaxy is not a flat disc but has a "warped" shape like a fascinator hat or a vinyl record that has been left in the sun. "The stars 60,000 light years away from the Milky Way's centre are as far as 4,500 [light years] above or below the galactic plane – this is a big percentage," said Dr Dorota Skowron of the University of Warsaw, first author of the latest research. Both the new study and an earlier one published in February, which found a similar shape, are based on the distribution across the galaxy of stars known as Cepheids – bodies whose brightness varies in a regular cycle over time. The new study, published in the journal Science, encompasses data from more than 2,400 Cepheids, allowing the team to build the most detailed three-dimensional map yet of the Milky Way. This means that we can use Cepheids to study the relatively recent history of our galaxy," said Skowron. The new research shows our galaxy, which Skowron notes has a radius of about 70,000 light years and four spiral arms, is "warped", with the outer regions of one side bent "up" from the galactic plane towards the north galactic pole, and the other bent "down".
These findings also tally with other work showing a warp and flare, including studies of the distribution of hydrogen atoms across the Milky Way, although Skowron notes that uncertain distances mean the shape of the warp could not be unpicked from those studies. Prof Richard de Grijs of Macquarie University, who co-authored the earlier Cepheid study, said there were a number of explanations for our galaxy's warp. These include mergers with smaller galaxies, or the gravitational pull of the Milky Way being weaker in its outer regions meaning bodies there might be deflected out of the galactic plane by the tug of other stars. Skowron said warping was not seen in the central regions of the Milky Way because gravitational forces there were so strong that it was hard to deflect stars and gas out of the galactic plane. As with the previous work, the new study shows the Cepheids disproportionately lie on one side of the warped galaxy, forming an arc-shaped spread.
"For the first time, our whole galaxy - from edge to edge of the disk - was mapped using real, precise distances," said University of Warsaw astronomer Andrzej Udalski, co-author of the study published in the journal Science. Until now, the understanding of the galaxy's shape had been based upon indirect measurements of celestial landmarks within the Milky Way and inferences from structures observed in other galaxies populating the universe. The new map was formulated using precise measurements of the distance from the sun to 2,400 stars called "Cepheid variables" scattered throughout the galaxy. "Cepheids are ideal to study the Milky Way for several reasons," added University of Warsaw astronomer and study co-author Dorota Skowron. The map showed that the galaxy's disk, far from flat, is significantly warped and varies in thickness from place to place, with increasing thickness measured further from the galactic center.
A better way to map the Milky Way would be to directly measure our distance to a large sample of stars strewn across the galactic disk. Using this strategy, a team of scientists from the Astronomical Observatory at the University of Warsaw has compiled the most accurate 3D map of the Milky Way to date. Among several other new findings, the updated 3D map shows the S-shaped structure of our galaxy's distorted stellar disk. As the new research shows, this warp starts at ranges greater than 25,000 light-years from the galactic core, and it gets more severe with distance. "Stars that are 60,000 light-years away from the Milky Way's center are as far as 5,000 light-years above or below the Galactic plane," she said. The new research also showed that the thickness of the Milky Way is variable throughout. To create the 3D map, Skowron and her colleagues charted the location of Cepheid variable stars. A sample of over 2,400 Cepheids was used to create the new map, the majority of which were identified with the Optical Gravitational Lensing Experiment (OGLE) survey, which monitors the brightness of nearly 2 billion stars. Also, the new map is more accurate than previous efforts because of the greater number of stars and the "very high purity" of the Cepheids samples, she said. In addition to illustrating the warped and twisted nature of our galaxy, the new map showed that the Cepheids are surprisingly packed together closely in space, and are of similar ages. "This is a clear indication that they were created together, in the same star-forming region in the narrow Milky Way arms," said Skowron.