Keith Hawkins, an assistant professor of astronomy at the University of Texas at Austin, used chemical cartography — also known as chemical mapping — to identify parts of the Milky Way’s spiral arms. His research, Published in Monthly Notices of the Royal Astronomical SocietyThis pioneering technique demonstrates the value of understanding the shape, structure and evolution of our home galaxy.
Chemical maps of the galaxy show how the elements in the periodic table are distributed throughout the Milky Way. It enables astronomers to determine the location of celestial objects based on their chemical composition rather than the light they emit. Although the concept of chemical cartography has been around for a long time, astronomers have only recently been able to obtain significant results from this technique. Thanks to powerful telescopes coming online.
“Just as the early explorers created better and better maps of our world, we are now creating better and better maps of the Milky Way,” says Hawkins. “Those maps reveal what we think is true, but still needs to be verified.”
We have known since the 1950s that the Milky Way is a spiral galaxy. However, its exact shape and structure, even the number of its arms, are the subject of ongoing investigation. We live inside our galaxy and cannot travel far enough to see it from an outsider’s perspective. “It’s like being in a big city,” Hawkins explains. “You can look around the buildings, you can see what street you’re on, but it’s hard to know what the whole city looks like unless you’re in an airplane flying over it.”
Our limited view of the Milky Way hasn’t stopped astronomers from creating well-informed models of it; Or artists from painting beautiful illustrations of it. “But,” says Hawkins, “I wanted to find out how accurate those models and depictions really were. And to see if chemical cartography could reveal a clearer picture of the Milky Way’s spiral arms.”
Mapping the Milky Way
A traditional way to map the Milky Way is to identify the density of young stars. As the Milky Way rotates, it compresses the dust and gas in its spiral arms, causing the birth of new stars. Hence, it is predicted that where the young players are plentiful, there is also a hand.
Astronomers can detect the light emitted by young stars. But sometimes dust clouds obscure the stars and make it difficult for even the best telescopes to observe their light. As a result, some parts of the Milky Way’s arms have yet to be discovered.
Chemical cartography helps astronomers fill in the missing pieces.
It does this by relying on an astronomical concept called “metallicity”. Metallicity refers to the ratio of metals to hydrogen on a star’s surface. In astronomy, any element on the periodic table that is not hydrogen or helium is called a “metal”. Young stars have more metals than older stars and therefore have a higher metallicity. This is because they formed later in the history of our universe when more metals existed.
After the Big Bang, only hydrogen, helium, and trace amounts of a few metals existed. In their cores, the first generation of stars converted hydrogen and helium into more and more complex metals (ie, heavier and heavier elements in the periodic table) and eventually died or exploded. But out of chaos comes life. These explosions eject metals into their surroundings, where they are used as building blocks for the next generation of stars.
As the cycle of star birth and destruction repeats, each successive generation of stars is richer in complex metals than the one before it, giving rise to higher and higher metallicities. In theory, the spiral arms of the Milky Way, which contain many young stars, should have a higher metallicity than the regions between them.
Comparing maps
To create his map, Hawkins identified the distribution of metal in the Milky Way. He focused on the area around our Sun where this data resides – a view out to 32,600 light-years. Areas rich in metal-rich material were expected to line the spiral arms, and areas poor in metal-rich material were expected to line the spaces between the arms.
When he compared his own map to others in the same region of the Milky Way, the spiral arms lined up against each other. What’s more, because Hawkins’ map identifies spiral arms based on the metallicity rather than the light emitted by young stars, it showed new regions that had previously gone uncharted.
“The spiral arms are rich in metals,” says Hawkins. This illustrates the value of chemical cartography in identifying the composition and formation of the Milky Way. It has the potential to completely transform our view of the galaxy.
The Gaia Space Telescope is revolutionizing the study of our galaxy
As our telescopes become more powerful, so does the promise of chemical cartography.
For his research, Hawkins analyzed data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the Gaia Space Telescope. The new data from Gaia (Data Release 3) was particularly insightful. Because Gaia offers the most accurate and comprehensive survey to date, including the chemical composition of the Milky Way.
Since its launch in 2013, Gaia has observed nearly two billion objects. Astronomers can now extend their research from thousands of objects to billions of objects and to a much larger area of the galaxy.
“The vast amount of data available from Gaia now allows us to do chemical cartography on a galactic scale,” says Hawkins. “Data on both the positions and their chemical composition of billions of stars were not available until recently.”
So far, Gaia has provided chemical data for the largest region of the Milky Way so far. However, this is still only one percent of the galaxy. As Gaia continues to survey the heavens and new telescopes come online, astronomers can increasingly use chemical cartography to understand the fundamental properties of our galaxy. These lessons can be applied to other galaxies and the universe as a whole. As Hawkins explains, “It’s a new era.”
Acknowledgments
Gaia is a European Space Agency (ESA) mission. The spacecraft is controlled by three ground stations at the European Space Operations Center (ESOC, Darmstadt, Germany): Cebreros (Spain), Malargüe (Argentina) and New Norcia (Australia). Scientific activities are carried out from the European Space Astronomy Center (ESAC, Villafranca, Spain). The Gaia Data Processing and Analysis Consortium (DPAC) processes the raw data published in one of the largest star catalogs ever produced.
The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.