Scientists use chemical mapping to study the spiraling arms of the Milky Way

The light emitted by such young stars allows astronomers to find them. Chemical cartography, on the other hand, can be employed in some regions when the view of the stars is hindered by cosmic dust. 

Chemical mapping uses an astronomical concept known as metallicity, which detects the "ratio of metals to hydrogen" present on the surface of a star. 

Young stars are considered to have a higher metal content than older stars. This implies that young stars will have a greater metallicity as well. 

“In theory, the Milky Way’s spiral arms, which contain an abundance of young stars, should have a higher metallicity than the regions between them,” noted the release.  

Hawkins created his map using the concept of metalicity. 

He particularly examined a region around the Sun roughly 32,600 light-years wide. The young stars’ data came from the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) and the Gaia satellite telescope.

He created the map based on the theory that metal-rich objects should line up with spiral arms, while those with less metal should fill the spaces in between the arms.

When he compared his map to others of the same area of the Milky Way, the existence of spiral arms, as predicted, lined up. 

Applications of this technique 

“A big takeaway is that the spiral arms are indeed richer in metals. This illustrates the value of chemical cartography in identifying the Milky Way’s structure and formation. It has the potential to fully transform our view of the Galaxy,” said Hawkins. 

Creating chemical maps may provide useful information about the distribution of elements across the Milky Way. This, in turn, may aid in determining the positions of numerous astronomical objects, such as stars and planets, depending on their chemical composition. 

Because we are located inside the Milky Way, astronomers have a limited view of it, making it difficult to explore those far-reaching spiral arms. 

“It’s like being in a big city. You can look around at the buildings and you can see what street you’re on, but it’s hard to know what the whole city looks like unless you’re in a plane flying above it,” explained Hawkins. 

Hawkins believes that with the advent of advanced telescopes, scientists will be able to generate a more exact and sophisticated chemical mapping of the galaxy.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.

Study abstract:

Chemical Cartography, or mapping, of our Galaxy has the potential to fully transform our view of its structure and formation. In this work, we use chemical cartography to explore the metallicity distribution of OBAF-type disk stars from the LAMOST survey and a complementary sample of disk giant stars from Gaia DR3. We use these samples to constrain the radial and vertical metallicity gradients across the Galactic disk. We also explore whether there are detectable azimuthal variations in the metallicity distribution on top of the radial gradient. For the OBAF-type star sample from LAMOST, we find a radial metallicity gradient of Δ[Fe/H]/ΔR ∼−0.078 ± 0.001 dex/kpc in the plane of the disk and a vertical metallicity gradient of Δ[Fe/H]/ΔZ ∼−0.15 ± 0.01 dex/kpc in the solar neighborhood. The radial gradient becomes shallower with increasing vertical height while the vertical gradient becomes shallower with increasing Galactocentric radius, consistent with other studies. We also find detectable spatially-dependent azimuthal variations on top of the radial metallicity gradient at the level of ∼0.10 dex. Interestingly, the azimuthal variations appear be close to the Galactic spiral arms in one dataset (Gaia DR3) but not the other (LAMOST). These results suggest that there is azimuthal structure in the Galactic metallicity distribution and that in some cases it is co-located with spiral arms.