The Milky Way in three dimensions

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A team of Polish astronomers from the Astronomical Observatory of the University of Warsaw working in the OGLE project have created a unique three-dimensional map of the Milky Way. The map presents a precise picture of our galaxy and provides lots of new information on the structure and history of the star system where we live. The achievement of the Polish researchers is described in the latest issue of the weekly Science. The authors of the work include several laureates of FNP programmes.

Astronomers have known since the 17th century that Earth, the Sun, and other planets from the solar system, along with the billions of stars visible through telescopes, make up our galaxy. The light from these stars, observed far from the lights of civilization, blends to form the shape of milk spilled in the heavens and thus the Milky Way. But it is not an easy task to describe the true shape and structure of the galaxy based on observations from within the galaxy.

Astronomers picture the Milky Way as a typical spiral galaxy composed of a central bulge with an oval “bar” structure surrounded by a flat disk made of gas, dust and stars. The disk comprises four spiral arms and has a diameter of about 120,000 light years. Our solar system is located inside the disk, about 27,000 light years from the centre of the galaxy. This is why the stars of the disk viewed from this location appear in the sky as a thin, pale glow: the belt of the Milky Way.

1. The Warsaw Telescope and Galactic Cepheids from OGLE (K. Ulaczyk J. Skowron OGLE)
Photo: Warsaw telescope and Cepheids in the Milky Way discovered by the OGLE sky survey (K. Ulaczyk/J. Skowron/OGLE/University of Warsaw Astronomical Observatory)

Current knowledge of the construction of our galaxy is based among other things on counting of stars, radio observations of the distribution of gas particles in the galaxy, and an analysis of external images of other galaxies. But so far, the distances from the objects used to describe the structure of the galaxy were also determined indirectly and were strongly dependent on the adopted models. Thus the most precise method for learning the structure of the galaxy would be to determine precise distances from a large group of stars with similar properties, thanks to which we could directly observe their distribution in the galaxy in three dimensions.

The ideal objects for mapping the Milky Way are relatively young stars (less than 250 million years old) known as classical Cepheids. They are pulsating supergiants whose brightness changes very regularly, with a period from several hours to several weeks.

As explained by Dorota M. Skowron, leader of the team preparing the map of the galaxy and lead author of the article, “Based on the period of pulsation, we can determine the true brightness of a Cepheid, and by comparing it to the observed brightness of the star we can precisely calculate its distance. One difficulty in obtaining precise results is the absorption of light on the way from the star to the observer on Earth, but astronomers deal with this problem by taking observations in the region of infrared radiation, where the absorption is very low. The distance to a Cepheid can be determined with a precision closer than 5%.”

The latest map of the galaxy by the OGLE team (Optical Gravitational Lensing Experiment) presented in the journal Science was created on the basis of data from over 2,400 Cepheids. Most of them are objects newly discovered thanks to observations conducted as part of the OGLE project at the Las Campanas Observatory in Chile.

According to Prof. Andrzej Udalski, director of the OGLE project and laureate of the Foundation for Polish Science Prize, “The OGLE project is one of the world’s largest photometric surveys of the sky, regularly observing over two billion stars. The collections of various types of variable stars, including Cepheids from our galaxy and the neighbouring Magellanic Clouds, are among the largest in contemporary astrophysics and are the basis for a range of studies of the universe.”

The map constructed on the basis of the Cepheids studied shows the true distribution of the young star population in the galaxy. It is the first 3D map created on the basis of directly determined distances to specific objects. The precise determination of the distances of the Cepheids filling up the galactic disk out to its edge enables a precise analysis of the construction of the galactic disk. The Sun is located about 50 light years above the plane of the disk. The map shows that the galactic disk is flat to a distance of 25,000 light years from the centre of the galaxy, but is twisted farther out, undergoing “disk warp.”

Przemysław Mróz, a doctoral student at the University of Warsaw and laureate of FNP’s START programme who is studying the parameters of the galactic disk, explained: “Disk warp was suspected many years ago, but only now can we use individual objects for the first time to study its shape in three dimensions.” Stars in the outer portions of the Milky Way may be shifted by as much as 4,500 light years from the plane of the disk mapped in the central regions of the galaxy. Disk warp may be caused by the influence of other galaxies, intergalactic gas, or dark matter.

The galactic disk does not have a uniform thickness. This is the first time that “disk flaring” has been characterized with such precision in the case of a young population of the galaxy’s stars. The thickness of the galactic disk is about 500 light years at the distance of the Sun and reaching over 3,000 light years at the very edges of the disk.

Determination of the precise distances to such a numerous sample of Cepheids, combined with measurements of their speeds from the satellite Gaia, has also made it possible to construct the precise curved rotation of the galaxy: the dependence of the orbital speed of stars around the centre of the galaxy on their distance from the centre.

Mróz added: “Our curved rotation of the galaxy far exceeds the scope of earlier research and confirms the constant orbital speed of stars, practically to the very edges of the disk.” This shape is one of the fundamental arguments for the existence of dark matter in our galaxy.

The age of Cepheids is correlated with their period of pulsation. On this basis, a tomography of the age of Cepheids from the galaxy can be performed. It turns out that a number of distinct structures visible on the map are of similar age. Younger Cepheids are located closer to the centre of the galaxy and older ones at its edges.

As Dr Jan Skowron, co-author of the article in Science, pointed out, “The similar age of structures indicates that they must have arisen at a similar time in the past in one of the spiral arms of the galaxy. Their present distribution in the disk and partial blurring is the result of the varied speed of rotation of the spiral arms in the galaxy (gas structures in which young stars, including Cepheids, arise), as well as the rotation of the stars.”

To test this hypothesis, a simple model of creation of specific structures was constructed. Episodes of formation of stars at various times in the past were positioned in the spiral arms of the galaxy, and typical own motions and rotation speed were assigned to the stars. It was then checked how the Cepheids arising millions of years ago in fragments of the spiral arms would be situated in today’s galaxy.

Jan Skowron summed up the results of the modelling as follows: “The simulated and observed structures in the galaxy are strikingly similar. We can thus state that our model for the history of the galactic disk is possible and can explain today’s structures as we observe them in the disk.”

The article “A three-dimensional map of the Milky Way using classical Cepheid variable stars,” presenting a 3D map of the galaxy and describing the results of the OGLE team, was published in the weekly Science. The authors are Dorota M. Skowron, Jan Skowron, Przemysław Mróz (FNP START laureate 2018), Andrzej Udalski (laureate of the SUBIN and MISTRZ programmes and the Foundation for Polish Science Prize), Paweł Pietrukowicz (START laureate), Igor Soszyński (POWROTY/HOMING laureate) , Michał K. Szymański, Radosław Poleski (START laureate), Szymon Kozłowski, Krzysztof Ulaczyk, Krzysztof Rybicki and Patryk Iwanek.

Further reading:

Illustrations above: The age of Cepheids in the Milky Way. The oldest stars (red) were created 400 million years ago, and the youngest (blue) 30 million years ago (J. Skowron/OGLE/University of Warsaw Astronomical Observatory)

Photo: Warsaw telescope and Cepheids in the Milky Way discovered by the OGLE sky survey (K. Ulaczyk/J. Skowron/OGLE/University of Warsaw Astronomical Observatory)