Astronomers have created the most detailed three-dimensional map of our galaxy, revealing the true shape of the Milky Way: warped and twisted. By measuring the distance from our sun to thousands of individual pulsating stars scattered across the Milky Way, researchers have charted our galaxy on a larger scale than ever before. The newly constructed three-dimensional map of the Milky Way - reported by Polish astronomers from the Astronomical Observatory of the University of Warsaw - 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, it is warped at distances greater than 25,000 light-years from the galactic center. It reveals the S-like structure of the Milky Way’s warped stellar disc (as seen in the image above by J. Skowron / OGLE / Astronomical Observatory, University of Warsaw).
According to scientists, warping may have been caused by past interactions with satellite galaxies, interactions with intergalactic gas or dark matter. “Our map shows that the Milky Way disk is not flat, it is warped and twisted far away from the galactic center. Warping of the disc has been detected before, but this is the first time we can use individual objects to trace its shape in three dimensions,” the researchers say in their findings, published in the journal Science. The research provides a crucial and the most detailed map till date for studies of our galaxy’s stellar motions and the origins of the Milky Way’s disk.
Scientists have so far believed that the Milky Way is a typical barred spiral galaxy comprising a bar-shaped core region surrounded by a flat disk of gas, dust, and stars. The Milky Way’s disk consists of four spiral arms and has a diameter of about 120,000 light-years. The solar system is located within the disk, about 27,000 light-years from the galactic center. However, much of the current understanding of the spiral shape and structure of our galaxy is built upon indirect measurements to celestial landmarks and inferences based on other galaxies in the universe. Accordingly, the galactic map drafted by those limited observations remains incomplete. The unique three-dimensional map thus gives new insights into the structure and history of our galaxy.
For many centuries, astronomers have been aware that the Earth, the sun and other planets in the solar system, together with billions of stars seen with telescopes, form our galaxy. These stars, if observed far from city lights, look like milk spilled across the sky and form the band of the milky way.
However, the internal structure and history of the Milky Way are still far from being well understood, in part because it is extremely difficult to measure distances to stars in the outer regions of the galaxy. The research team says that fortunately, there is a type of variable stars called classical Cepheids, which are bright enough to be seen out to the edge of our galaxy. The three-dimensional map of the Milky Way, presented by the researchers, is based on a precise distance of thousand of these stars.
Cepheids are young, pulsating supergiant stars and their brightness changes in a very regular pattern, with a well-defined period, which may range from hours to several dozen days. “Classical Cepheids are young (less than 400-million-years-old) supergiant stars, whereas other types of Cepheids (type II and anomalous) arise in older stellar populations. The intrinsic luminosities of classical Cepheids span the range of 100 to 10,000 solar luminosities. This is bright enough to be detected at extragalactic distances and, within our galaxy, through obscuring interstellar clouds of gas and dust in the foreground,” says the paper.
The team used their telescope located at Las Campanas Observatory in Chile to image the entire visible Milky Way over a hundred times. They then searched for stars that change their brightness in a specific pattern. “Cepheids are ideal for studying the Milky Way structure because they follow the relation between their pulsation period and their luminosity, meaning that we can measure their intrinsic brightness based on their period. The distance can then be determined by comparing the apparent and intrinsic brightness of the star. Distances to Cepheids can be measured with an accuracy better than 5%,” say the researchers in their findings.
The new three-dimensional map 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. By determining the 3-D coordinates of each distant pulsing star relative to our sun, the team built a largescale 3-D model of the Milky Way galaxy. The OGLE survey is among the largest sky variability surveys, which monitors the brightness of almost two billion stars. OGLE collections of variable stars, including Milky Way Cepheids, are the largest in the world and many researchers use it studying the universe.
“The Milky Way is a barred spiral galaxy, with physical properties inferred from various tracers informed by the extrapolation of structures seen in other galaxies. However, the distances of these tracers are measured indirectly and are model-dependent. We constructed a map of the Milky Way in three dimensions, based on the positions and distances of thousands of classical Cepheid variable stars. This map shows the structure of our galaxy’s young stellar population and allows us to constrain the warped shape of the Milky Way’s disk. A simple model of star formation in the spiral arms reproduces the observed distribution of Cepheids,” says the study.
Findings show that the Galactic disk does not have a constant thickness; it flares with the increasing distance from the Galactic center. The Galactic disk is about 500 light-years thick near the sun, whereas over 3,000 light-years near its edge.
The astronomers also estimated the age of a Cepheid based on its pulsation period. The youngest Cepheids, according to the team, are located near the galactic center, while the eldest, near the Milky Way’s edge. “We found many elongated structures in the galactic disk that are composed of stars of similar age. We propose that stars located in those sub-structures 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,” says the study.
The researchers conducted a simple simulation to test such a hypothesis. They injected several star formation episodes into the spiral arms and assigned typical motions and rotation velocities to the stars within them. “We then look at what the galaxy looks like after millions of years to find that 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,” the results reveal.
In a previous study, astronomers from Macquarie University and the Chinese Academy of Sciences had used 1339 ‘standard’ stars to map the real shape of our home galaxy. Their study, published in Nature Astronomy, said that the Milky Way’s disc of stars becomes increasingly ‘warped’ and twisted the farther the stars are from the galaxy’s center. This map showed that the warped Milky Way disc also contains young stars, thus confirming that the warped spiral pattern is caused by torque from the spinning of the Milky Way’s massive inner disc of stars.