Our Milky Way Galaxy hosts an inner ring-like structure that is rich in metals (elements heavier than hydrogen and helium) and intermediately aged, according to an analysis of stellar data from the APOGEE Survey.
Wylie et al. investigated the metallicity, age, and orbital anatomy of the inner Milky Way, specifically focusing on the outer bar region. This image shows a metallicity map 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 dashed lines mark different sight lines. The red contours 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 fur Extraterrestrische Physik.
APOGEE is a large-scale, stellar spectroscopic survey conducted at near-infrared wavelengths.
As opposed to optical light, infrared light can more easily pierce though dust, allowing APOGEE to detect stars located in the dusty regions of the Milky Way, such as the disk and bulge, and determine not only their element abundances but also their positions, line-of-sight velocities, and approximate ages.
In addition, ESA’s Gaia mission is charting about one billion stars, providing positional and proper motion measurements.
Together both surveys provide all the necessary observational ingredients to determine orbits of stars in the inner regions of the Milky Way.
All that is needed is a realistic Milky Way potential to integrate the stars in.
This is obtained from the inner Milky Way model created by MPE scientists.
“We integrated more than 30,000 stars from the APOGEE Survey with additional data from Gaia in our Milky Way bar-bulge potential to obtain the full orbits of these stars,” said Shola Wylie, a Ph.D. student at the Max-Planck-Institut fur Extraterrestrische Physik.
“And with these orbits, we can effectively see behind the galactic bulge as well as other spatial regions not covered by the surveys.”
Wylie and colleagues then used these orbits to build maps of the stellar density, metallicity and age for 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 have younger ages, around 6 billion years,” Wylie said.
While star forming inner rings have been seen in other disk galaxies, it was not clear that our home Galaxy contains a stellar inner ring.
To separate the stars in the ring and the bar structures, the astronomers used the eccentricity of the orbits, i.e. how much the orbit deviates from a circle.
They found 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 inflowing gas after the bar was in place,” said Dr. Ortwin Gerhard, also from the Max-Planck-Institut fur Extraterrestrische Physik.
Therefore the age of the stars in the inner ring can be used to look back at the formation history of the Milky Way.
“It is still unclear if there is a connection between the Galaxy’s inner ring and its spiral arms and whether gas is currently funneled inwards to a star forming, thin inner ring such as 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 further data.”
The results appear in the journal Astronomy & Astrophysics.
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Shola M. Wylie et al. 2022. The Milky Way’s middle-aged inner ring. A&A 659, A80; doi: 10.1051/0004-6361/202142343
