04Aug2019

Study Finds Our Galaxy Is ‘Warped And Twisted’

A team of Polish astronomers has created the most accurate three-dimensional map of the Milky Way to date, revealing surprising distortions and irregularities along the galactic disc.

The Milky Way is not flat like a pancake, and is surprisingly twisted. (ly = light-year) (Image: J. Skowron/OGLE/Astronomical Observatory, University of Warsaw)

Building an accurate map of the Milky Way is not easy. Our location deep inside the gigantic structure means we can’t observe our galaxy externally, forcing us to envision its form from within. Dense expanses of stars, gas, and dust complicate our view even further. Despite these limitations, we know that the Milky Way is a spiral galaxy measuring around 120,000 light-years across, and that we’re located around 27,000 light-years from the galactic core.

Over the years, astronomers have employed various attempts to map our galaxy, including star counts, radio observations of gas molecules, and even extrapolations of similar spiral galaxies nearby. These efforts have resulted in maps that are decent but not great. 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. Better yet, the stars used in this sample would belong to a specific, well-studied type to ensure observational accuracy.

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. Astronomer Dorota Skowron led the study, which was published Friday in Science.

Animation showing the twisted shape of our galaxy. (Gif: J. Skowron/OGLE/Astronomical Observatory, University of Warsaw/Gizmodo)
Among several other new findings, the updated 3D map shows the S-shaped structure of our galaxy’s distorted stellar disk. The Milky Way is not flat like a pancake, and is instead “warped and twisted,” in the words of co-author Przemek Mroz, who described his team’s work in a related video.

That our galaxy is warped was already known, but the new research further characterises the surprising extent of these distortions. 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.

“If we could see our galaxy from the side, we would clearly see its warp,” Skowron told Gizmodo. “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. “This is a big percentage.” In terms of a cause, the researchers attributed interactions with neighbouring galaxies, intergalactic gas, and possibly even dark matter.

The new research also showed that the thickness of the Milky Way is variable throughout. Our galaxy gets thicker with distance from the core. At our location, for example, the galactic disk is about 500 light-years thick, but at the outer edges it’s as much as 3000 light-years thick.

Milky Way Cepheids on the Milky Way map. (Image: J. Skowron/Serge Brunier)
To create the 3D map, Skowron and her colleagues charted the location of Cepheid variable stars. These young, pulsating supergiants are ideal for this research because their brightness changes in a very regular pattern. Ultimately, the location of Cepheid stars within the Milky Way can be more accurately pinned down than other kinds of stars, which is precisely what was needed for this mapping project.

A sample of over 2400 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. In total, the researchers observed the galactic disk for six years, taking 206,726 images of the sky.

The 1.3 metre Warsaw telescope in the Chilean Andes is used for the OGLE survey, and it can monitor the brightness of stars and measure their properties for years. This makes the map more spatially complete than maps produced by data from the Gaia satellite, for example, in which distances are reliable only to 10,000 to 15,000 light-years, said Skowron. 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.

“So this is the most ‘real’ map of the Milky Way,” Skowron told Gizmodo.

If this work sounds familiar, it’s because research published earlier this year in Nature Astronomy employed a similar technique, in which scientists from the Chinese Academy of Sciences reached similar conclusions, using a different group of Cepheids for their map. One of the scientists behind the previous research, Xiaodian Chen from the National Astronomical Observatories at the Chinese Academy of Sciences, took issue with the fact that the authors of the new paper did not cite his team’s work. Nonetheless, he still liked the new science.

“They essentially confirmed our earlier conclusions regarding the 3D shape of the Milky Way’s disk, including its flaring in the outer regions,” Chen said. “A good thing about their confirmation of our work is that they used a different data set, covering 2,431 Cepheids compared to [our] 2,330, observed with a different telescope and through different filters. Yet they found pretty much the same result, which is comforting!”

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. “We can see with our own eyes, and inside our own galaxy, that star formation is not a constant process, but indeed is happening in bursts.”

The new map will also help to further clarify the physical structure of the Milky Way, the number of spiral arms (which is still debated), and the severity of the arms’ spiral twist, among other aspects. To improve the current map, Skowron said observations made from the Northern Hemisphere would add further clarity, along with the use of observatories capable of peering through to the other side of the galactic core and at super-dusty regions very close to the galactic plane. Infrared telescopes, she said, could do the trick.

Looking ahead, Skowron is hoping to chart older stars, which would allow them to visualise the evolution of the Milky Way over time.

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