A new map of the Milky Way in three dimensions shows us that its twisted

This diagram shows a warped shape of a galaxy like our Milky Way as suggested by observations of young variable stars, called Cepheids, our own galaxy. Credit: J. Skowron/OGLE/Astronomical Observatory, University of Warsaw.

Since Galileo pointed his telescope to the heavens and saw the Milky Way in the early 17th century discovering the fact that our galaxy consists of countless stars, studying the properties and history of our Galaxy has absorbed many generations of scientists. Now a group of Polish astronomers from the Astronomical Observatory of the University of Warsaw have presented a unique three-dimensional map of the Milky Way, providing insights into the structure and history of our Galaxy.

Astronomers have been aware of the Earth, the Sun, and other planets in the Solar System together with billions of stars seen with telescopes, since the 17th century. Those stars when observed in good condition in a place with no light pollution, they look like milk spilled across the sky and form the band of the Milky Way.

Currently, it is believed that the Milky Way is a typical barred spiral galaxy with a bar-shaped core region surrounded by a flat disk of gas, dust, and stars. This disk has spiral arms with a diameter of about 120,000 light years. Our own Solar System is located about 27,000 light years from the Galactic centre, that is the reason why the disk of stars, viewed from within, look like a faint band in the sky, the Milky Way. 

Our current model for what the shape of the Milky Way disk looks like is based on various tracers (such as star counts or radio observations of gas molecules) this is informed by the extrapolation of structures seen in other galaxies. However, the distances to these tracers are measured indirectly and are model-dependent. The most reliable method for studying the shape of the Milky Way would be to directly measure distances to a large sample of stars of a specific type, which would allow one to construct the three-dimensional map of the Galaxy.

This diagram shows a view of our Milky Way, as well as a plot of variable Cepheid stars that shows the galaxy's warped shape. Credit: Plot by J. Skowron/OGLE, the Milky Way panorama by Serge Brunier.

Doctor Dorota Skowron, the lead author of the study said: “Cepheids follow a relation between their pulsation period and luminosity, allowing the intrinsic luminosity of a Cepheid to be inferred from its period. The distance can then be determined by comparing the apparent and intrinsic brightness,”. She added: The additional difficulty arises from the presence of interstellar dust and gas which may dim the brightness of a Cepheid. Fortunately, observations in infrared bands reduce these uncertainties.

This new map in three dimensions of the Milky Way has been constructed by using a sample of over 2,400 Cepheids, the majority of which have been newly identified in the photometric data collected by the Optical Gravitational Lensing Experiment (OGLE) survey.

This image shows a view of the Warsaw Telescope, with the lighthouse-like Cepheid stars found in the OGLE survey marked in yellow. Credit: K. Ulaczyk / J. Skowron / OGLE / Astronomical Observatory, University of Warsaw.

Professor Andrzej Udalski, the PI of the OGLE project said. “The OGLE survey is among the largest sky variability surveys; it monitors the brightness of almost two billion stars. OGLE collections of variable stars, including Milky Way Cepheids, are the largest in the world and are used by many researchers for various studies of the Universe.” 

This new three-dimensional made map of the Milky Way is the first map that is based on direct distances to thousands of individual objects as distant as the expected boundary of the Galactic disk. The map demonstrates that the Milky Way disk is not flat, but instead it is warped at distances greater than 25,000 light years from the Galactic centre.

Przemek Mróz, a graduate student at the University of Warsaw said that: “Warping of the Galactic disk has been detected before, but this is the first time we can use individual objects to trace its shape in three dimensions.” The stars in the outer parts of the Milky Way disk may be displaced from the Galactic plane by 4,500 light years relative to the central regions of the Galaxy. Warping may have been caused by interactions with satellite galaxies, intergalactic gas, or dark matter.

The Galactic disk does not have a constant thickness, instead, it flares with the increasing distance from the Galactic centre. The Galactic disk is about 500 light years thick near the Sun, whereas over 3,000 light years near its edge. The age of Cepheid stars can be determined based on their pulsation periods which has allowed astronomers to perform an age tomography of the Milky Way. The youngest Cepheids are located near the Galactic center, while the eldest – near the Milky Way’s edge.

This image compares a simulation of the Milky Way galaxy's Cepheid star variables (left) with actual observations of their numbers (right). Red points indicate older stars, while younger ones are shown in blue. Credit: J. Skowron/OGLE/Astronomical Observatory, University of Warsaw.

Doctor Jan Skowron, the coauthor of the study, explained said: “We found many elongated substructures in the disk composed of stars of similar age. This indicates that Cepheids located there must have formed around the same time in one of the spiral arms. However, Cepheids that were formed in a spiral arm do not currently follow the exact location of that arm, because rotation velocities of spiral arms and stars are slightly different.” 

Astronomers performed a simple simulation to test this hypothesis. They injected several star formation episodes into the spiral arms and assigned typical motions and rotation velocities to the stars within them.

“The simulated and observed structures are strikingly similar. This shows that our idea about the recent history of the Galactic disk is plausible and can explain the structures we see,” sums up Dr. Jan Skowron.

3D Map of the Milky Way, the Best View Yet of Our Galaxy's Warped and Twisted Shape. Credit: K. Ulaczyk / J. Skowron / OGLE / Astronomical Observatory, University of Warsaw.

The paper that describing the discovery has been published in Science under the title: “A three-dimensional map of the Milky Way using classical Cepheid variable stars“, D. M. Skowron, J. Skowron, P. Mróz, A. Udalski, P. Pietrukowicz, I. Soszyński, M. K. Szymański, R. Poleski, S. Kozłowski, K. Ulaczyk, K. Rybicki, and P. Iwanek 2019, Science, doi:10.1126/science.aau3181.

Source: University of Warsaw,