The Milky Way has an inner ring, just outside the core

Over the past century, astronomers have learned a great deal about the cosmos and our place in it. From the discovery that the universe is in a perpetual state of expansion to the discovery of the cosmic microwave background (CMB) and the cosmological model of the Big Bang, our perception of the cosmos has expanded immensely. And yet many of the most profound astronomical discoveries are still taking place in our cosmic backyard—the Milky Way.

Compared to other galaxies that astronomers can resolve with relative ease, the Milky Way’s structure and size are the subject of ongoing discoveries. The latest comes from the Max Planck Institute for Extraterrestrial Physics (MPE), where scientists have found a previously undiscovered inner ring of metal-rich stars just outside the galactic bar. The existence of this ring has provided new insights into star formation in this region of the galaxy during its early history.

Determining the structure and size of the Milky Way has always been hampered by the fact that we are within the Milky Way’s galactic disc near one of its spiral arms. From this vantage point, stars are obscured by dense clouds of gas and dust, especially toward the center of the Milky Way. This has made it particularly difficult to determine the structure of the inner Milky Way.

Using information from Gaia’s second data release, a team of scientists have made refined estimates of the Milky Way’s mass. Source: ESA/Gaia/DPAC

A remaining mystery about our galaxy is whether or not it had any star-forming inner rings seen in other disk galaxies. Fortunately, scientists at MPE have spent the last decade combining data from different observing campaigns – including the APOGEE survey and the Gaia Observatory – with advanced computer simulations. The result was a state-of-the-art model of the inner Milky Way, showing a slow bar with a peanut-shaped bulge. This bulge is populated by stars that formed four to nine billion years ago, with a maximum age of between six and eight billion years.

The APOGEE survey is a large-scale stellar spectroscopy campaign being conducted by the Sloan Digital Sky Survey (SDSS) at Apache Point Observatory in New Mexico. This study was performed at near-infrared wavelengths, allowing for observations that would not be possible with optical light. In particular, APOGEE’s IR observations allow it to see through the dusty regions of the Milky Way, such as the disk and the bulge.

This allowed the MPE team to determine the element abundances, positions, line-of-sight velocities and approximate ages of all stars in the newly observed bulge. In the meantime, the data have been obtained from the ESAs gaia Mission provided accurate measurements of the positions and proper motions of these stars. The team then combined all of these observations with a model they had built of how the inner Milky Way worked. When Shola M. Wylie, a Ph.D. Student at MPE and lead author of the study, explained:

“We integrated more than 30,000 stars from the APOGEE survey with additional data from Gaia into our Milky Way bar-bulge potential to obtain the full orbits of these stars. And with these orbits, we can effectively see beyond 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 in which the stars are of younger age, around 7 billion years.”

Annotated diagram of the Milky Way. The sun is indicated at the bottom of Orion’s spur. Photo credit: NASA

To separate the stars in the ring and the bar structures, the team observed how much their orbits deviated from a circle (ie their eccentricity). From this they found that the stars in the ring are younger and richer in metal than the stars in the bar and are more concentrated on the galactic plane. This suggests that stars in the star ring must have continued to form from inflowing gas after the bar was in place.

Therefore, astronomers can look back at the formation history of the Milky Way based on the age of the inner ring stars. Based on the average age of the stars, the MPE team estimates that the Galactic Bar formed at least 7 billion years ago. It’s not currently clear if there’s a connection between the newly discovered inner ring and the galaxy’s spiral arms, and if gas is currently being channeled inward to a star-forming thin inner ring like other spiral galaxies do.

As next-generation telescopes become operational, more detailed galactic surveys will be possible. Combined with advanced models (which will be possible with more sophisticated software), these data will allow astronomers to learn more about how the ring structure blends into the surrounding disk in the Milky Way. The study, titled “The Milky Way’s middle-aged inner ring,” describing their findings, was recently published in the journal Astronomy & Astrophysics.

Further reading: Max Planck Institute for Extraterrestrial Physics The Milky Way has an inner ring, just outside the core

Russell Falcon

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