December 9, 2023

Keith Hawkins of the University of Texas at Austin has produced a new chemical map of the Milky Way’s local region, revealing new structural details of the nearby spiral arms of the Milky Way, 32,600 light-years from Earth.

Since the 20s, scientists have known that the sun sits on the edge of one of the trillions of galaxies in the universe. However, figuring out the structure of the Milky Way is not easy.

It’s relatively easy to understand the structure of other galaxies because we can observe them from afar, but we’re in a galaxy’s disk about 30,000 light-years from the edge. This makes it very difficult to understand many details other than the shape of our nearby spiral arms and the fact that it is a barred spiral galaxy. Meanwhile, the wide arc of the far side of the Milky Way is hidden from us directly because of all the gas and dust obscuring it.

To learn more, Hawkins turned to chemical cartography, which locates objects in the Milky Way galaxy based on their chemical composition. Using data on the chemical composition of two billion objects provided by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and the Gaia space telescope, he was able to see objects invisible only with light.

Hawkins specifically looks for young stars marked by metallicity, or stars with higher concentrations of elements heavier than hydrogen and helium. These heavy elements are more prevalent in young stars than in old ones, because more of these elements are produced as the universe ages.

The first stars in the universe after the Big Bang consisted almost entirely of hydrogen and helium, with some added trace elements. As old stars die, especially when they explode as supernovae, there are more heavy elements available for new stars to absorb. Since new stars tend to form at the leading edges of galaxies’ spiral arms, where gas and dust are concentrated like passing waves, looking for their chemical signatures can reveal details about the structure of the spiral arms. At the same time, the absence of these signatures indicates a gap between the arms.

“An important conclusion is that the spiral arms are indeed metal-rich,” Hawkins said. “This illustrates the value of chemical mapping in identifying the structure and formation of the Milky Way. It has the potential to completely change the way we think about the Milky Way.”

The study was published in Royal Astronomical Society Monthly Notice.

source: macdonald observatory