Inner ring of the Milky Way has an average age

(ORDO NEWS) — Using a combination of observed stars and a realistic model of the Milky Way, scientists at the Max Planck Institute for Extraterrestrial Physics have discovered a new structure in our home galaxy.

Just outside the galactic bar (an elongated structure of stars and gas in the galactic disk), they found an inner ring of metal-rich stars that are younger than the stars in the bar.

The age of the ring stars suggests that the bar formed at least 7 billion years ago. The existence of this ring makes it likely that star formation from influx gas played an important role in these early epochs.

Understanding the global structure of our own galaxy is complicated by the fact that we are located near one of its spiral arms in the disk plane. In many directions, the stars are obscured by dense clouds of gas and dust.

This is especially true at the center of the Milky Way, which makes the Milky Way’s inner structure particularly difficult to reach.

However, over the past decade, scientists at the Max Planck Institute for Extraterrestrial Physics (MPE) have been able to combine data from various observational campaigns with sophisticated computer simulations to create a state-of-the-art model of the inner Milky Way: a slow bar with a peanut-shaped bulge.

Recent studies have yielded a wealth of new data about the inner Milky Way. APOGEE is a large-scale spectroscopic study of stars conducted in the near infrared wavelengths.

Unlike optical light, infrared light penetrates dust more easily, 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 elemental composition, but also their position, speed within line of sight and approximate age.

In addition, Gaia’s ambitious mission maps nearly a billion stars, providing positioning and proper motion measurements. Together, both surveys provide all the necessary observational ingredients to determine the orbits of stars in the inner regions of the Milky Way.

All that is needed is the realistic potential of the Milky Way for stellar integration. It comes from a model of the inner Milky Way created by MPE scientists.

“We combined over 30,000 stars from the APOGEE survey with additional data from Gaia in the bar-bulge potential of the Milky Way to obtain the complete orbits of these stars,” explains Shola M. Wylie, MPE graduate student and lead author of the study.

“Thanks to these orbits, we can effectively observe the galactic bulge, as well as other spatial regions not covered by research.” The scientists then used these orbits to plot stellar density, metallicity and age maps for the Milky Way’s interior.

“Around the central bar, we found an inner ring structure that is richer in metals than the bar, and where the stars are younger, about 7 billion years old,” she continues. Although star-forming inner rings have been seen in other disk galaxies, it was unclear whether our home galaxy has a stellar inner ring.

To separate the stars in the ring from the bar structures, the scientists used the eccentricity of the orbits, that is, how much the orbit deviates from a circle.

They found not only that the stars in the ring are younger and richer in metals than the stars in the bar, but that these stars are more concentrated in the direction of the galactic plane.

“The stars in the stellar ring must have continued to form from the inflowing gas after the bar was in place,” explains Ortwin Gerhard, lead scientist in the MPE dynamics group.

Therefore, the age of the stars in the inner ring can be used to study the history of the formation of the Milky Way: according to scientists, the galactic bar formed at least 7 billion years ago.

It is not yet clear whether there is a connection between the newly discovered inner ring and the spiral arms of the galaxy, or whether gas is currently being entrained within the star-forming thin inner ring, as is observed in other spiral galaxies.

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

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