The Milky Way has an Inner Ring, Just Outside the Core

Astronomers have learned a lot about our place in the universe. Our perception of the universe has expanded greatly since we discovered that the universe is in a constant state of expansion. Many of the most profound discoveries still occur in the Milky Way.

The structure and size of the Milky Way have been the subject of ongoing discovery compared to other galaxies. A previously undiscovered inner ring of metal-rich stars has been found by scientists at the Max Planck Institute for Extraterrestrial Physics. The ring has revealed new information about the star formation in this region of the galaxy.

The fact that we are located close to one of the spiral arms of the Milky Way makes it difficult to determine its structure and size. Clouds of gas and dust obscure stars from view from this vantage point. It is difficult to determine the structure of the inner Milky Way.

There is a mystery about whether or not our galaxy had any star-forming inner rings, which have been seen in other disk galaxies. Scientists at the MPE combined data from various observation campaigns over the past decade with advanced computer simulations. A state-of-the-art model of the inner Milky Way revealed a slow bar with a peanut-shaped bulge. The bulge is filled with stars that formed four to nine billion years ago, with a peak in age between six and eight billion years.

The Apache Point Observatory in New Mexico is the location of the APOGEE survey. The survey was conducted at near-infrared wavelength, which allows for observations that would not be possible in optical light. The IR observations allow it to see through the dusty regions of the Milky Way, such as the disk and bulge.

The abundance element, positions, line-of-sight velocities, and approximate ages of all the stars in the newly-observed bulge were determined by this. The data obtained by the Gaia mission provided accurate measurements of the positions and motions of the stars. The team created a model of the workings of the inner Milky Way. The lead author of the study was a student at MPE.

“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. And with these orbits, we can effectively see behind the galactic bulge as well as other spatial regions not covered by the surveys. 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 7 billion years.”

The team observed how much the stars deviated from a circle to separate them from the ring and bar structures. The stars in the ring are younger and more metal-rich than the stars in the bar, and they are more focused towards the Galactic plane. The stars in the stellar ring must have formed from inflowing gas after the bar was in place.

Astronomers can use the age of the inner ring stars to look back at the formation of the Milky Way. The Galatic bar is thought to have formed at least 7 billion years ago, based on the average age of the stars. It is not clear if there is a connection between the newly discovered inner ring and the galaxy's spiral arms and whether gas is currently funneled inwards to a star-forming thin inner ring.

More detailed surveys will be possible with next- generation telescopes. Astronomers will be able to learn more about how the ring structure transitions to the surrounding disk when combined with augmented models. The study that describes their findings was published in the journal Astronomy and Astrophysics.

There is a Max Planck Institute for Extraterrestrial Physics.