The Milky Way’s inner ring is nearly 6 billion years old, astronomers say

According to an analysis of stellar data from the APOGEE survey, our Milky Way galaxy hosts a ring-shaped internal structure rich in metals (elements heavier than hydrogen and helium) and of intermediate age.

wylie et al. studied the metallicity, age, and orbital anatomy of the inner Milky Way, focusing specifically on the outer bar region. This image shows a map of the metallicity of the inner Milky Way, based on orbital data calculated by Wylie et al. The red star marks the position of the Sun, while the white dotted lines mark different lines of sight. Red outlines show specific density levels to highlight important features: a bar in the middle with a ring-like structure around it. Image credit: Max-Planck-Institut für Extraterrestrische Physik.

APOGEE is a large-scale stellar spectroscopic study conducted at near-infrared wavelengths.

Unlike optical light, infrared light can more easily pierce dust, allowing APOGEE to detect stars located in dusty regions of the Milky Way, such as the disk and bulge, and determine not only their abundances of elements but also their positions, their line-of-sight speeds and approximate ages.

In addition, ESA’s Gaia mission maps approximately one billion stars, providing measurements of position and proper motion.

Together, the two surveys provide all the observational ingredients needed to determine the orbits of stars in the inner regions of the Milky Way.

All it takes is a realistic potential for the Milky Way to embed the stars in it.

This is obtained from the internal model of the Milky Way created by MPE scientists.

“We’ve integrated more than 30,000 stars from the APOGEE survey with additional data from Gaia into our potential Milky Way bar bulge to get the full orbits of these stars,” said Shola Wylie, registrar. a Ph.D. student at the Max-Planck-Institut für Extraterrestrische Physik.

“And with those orbits, we can actually see behind the galactic bulge as well as other regions of space not covered by the surveys.”

Wylie and his colleagues then used these orbits to construct maps of the stellar density, metallicity and age of the inner Milky Way.

“Around the central bar we found an inner ring structure that is more metal-rich than the bar and where the stars are younger in age, around 6 billion years old,” Wylie said.

Although star-forming inner rings have been observed in other disk galaxies, it was not clear that our home galaxy contained a stellar inner ring.

To separate the stars in the ring and the bar structures, astronomers have used the eccentricity of the orbits, that is, how far the orbit deviates from a circle.

They discovered not only that the stars in the ring are younger and more metal-rich than the stars in the bar, but also that these stars are more concentrated towards the galactic plane.

“Stars in the stellar ring must have continued to form from the influx of gas after the bar had been put in place,” said Dr Ortwin Gerhard, also of the Max-Planck-Institut für Extraterrestrische Physik.

Therefore, the age of stars in the inner ring can be used to trace back the history of the formation of the Milky Way.

“It is still unclear whether there is a connection between the Galaxy’s inner ring and its spiral arms and whether the gas is currently funneled into the interior of a thin, star-forming inner ring, as reported. seen in other spiral galaxies,” the authors said.

“Further work is needed to better understand the transition from the ring to the surrounding disk in the Milky Way, requiring augmented models and additional data.”

The results appear in the newspaper Astronomy & Astrophysics.

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Shola M.Wylie et al. 2022. The middle-aged inner ring of the Milky Way. A&A 659, A80; doi: 10.1051/0004-6361/202142343

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