Dubbed Shakti and Shiva, the newly-identified structures are between 12 and billion years old — so ancient they likely formed before even the oldest parts of the present-day Milky Way’s spiral arms and disk.

This image shows the location and distribution of Shakti (yellow) and Shiva (blue) stars throughout the Milky Way. Image credit: ESA / Gaia / DPAC / K. Malhan.

“What’s truly amazing is that we can detect these ancient structures at all,” said Dr. Khyati Malhan, an astronomer at the Max Planck Institute for Astronomy.

“The Milky Way has changed so significantly since these stars were born that we wouldn’t expect to recognize them so clearly as a group — but the unprecedented data we’re getting from ESA’s Gaia satellite made it possible.”

Using Gaia observations, Dr. Malhan and Max Planck Institute for Astronomy’s Dr. Hans-Walter Rix were able to determine the orbits of individual stars in the Milky Way, along with their content and composition.

“When we visualised the orbits of all these stars, two new structures stood out from the rest among stars of a certain chemical composition. We named them Shakti and Shiva,” Khyati said.

Each stream contains the mass of about 10 million Suns, with stars of 12 to 13 billion years in age all moving in very similar orbits with similar compositions.

The way they’re distributed suggests that they may have formed as distinct fragments that merged with the Milky Way early in its life.

Both Shakti and Shiva streams lie towards the Milky Way’s heart.

Gaia explored this part of the Milky Way in 2022 using a kind of galactic archaeology. This showed the region to be filled with the oldest stars in the entire Galaxy, all born before the disk of the Milky Way had even properly formed.

“The stars there are so ancient that they lack many of the heavier metal elements created later in the Universe’s lifetime,” Dr. Rix said.

“The stars in our Galaxy’s heart are metal-poor, so we dubbed this region the Milky Way’s ‘poor old heart’.”

“Until now, we had only recognized these very early fragments that came together to form the Milky Way’s ancient heart.”

“With Shakti and Shiva, we now see the first pieces that seem comparably old but located further out.”

“These signify the first steps of our Galaxy’s growth towards its present size.”

While very similar, the two streams are not identical. Shakti stars orbit a little further from the Milky Way’s center and in more circular orbits than Shiva stars.

Fittingly, the streams are named after a divine couple from Hindu philosophy who unite to create the Universe.

Some 12 billion years ago, the Milky Way looked very different to the orderly spiral we see today.

We think that our Galaxy formed as multiple long, irregular filaments of gas and dust coalesced, all forming stars and wrapping together to spark the birth of our Galaxy as we know it.

It seems that Shaki and Shiva are two of these components — and future Gaia data releases may reveal more.

The authors also built a dynamical map of other known components that have played a role in our Galaxy’s formation and were discovered using Gaia data.

These include Gaia-Sausage-Enceladus, LMS1/Wukong, Arjuna/Sequoia/I’itoi, and Pontus.

These star groups all form part of the Milky Way’s complex family tree, something that Gaia has worked to build over the past decade.

“Revealing more about our Galaxy’s infancy is one of Gaia’s goals, and it’s certainly achieving it,” said Gaia project scientist Dr. Timo Prusti, an astronomer at ESA.

“We need to pinpoint the subtle yet crucial differences between stars in the Milky Way to understand how our Galaxy formed and evolved.”

“This requires incredibly precise data — and now, thanks to Gaia, we have that data.”

“As we discover surprise parts of our Galaxy like the Shiva and Shakti streams, we’re filling the gaps and painting a fuller picture of not only our current home, but our earliest cosmic history.”

The study was published in the Astrophysical Journal.


Khyati Malhan & Hans-Walter Rix. 2024. Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way. ApJ 964, 104; doi: 10.3847/1538-4357/ad1885

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