Newly detected stellar aggregations in the Milky Way could be the remnants of protogalaxies from its formation — or just resonances with the galaxy’s bar.

Shiva and Shakti stars distributed throughout the Milky Way
This image shows the location and distribution of stars that are part of the Shakti (yellow) and Shiva (blue) streams in the Milky Way. These streams of stars were identified by their shared orbital properties.
ESA / Gaia / DPAC / K. Malhan

Astronomers have identified two groupings of stars in the inner Milky Way that they conclude represent two early proto-galaxies that collided with an early version of the Milky Way, helping to build it into the large whirlpool of stars that we live in today. If their interpretation is correct, it helps give credence to the idea that galaxies first formed from collisions of many smaller aggregations of gas and stars. The finding may thus provide glimpses of our galaxy in its earliest stages of formation — in essence, our galaxy’s baby pictures.

The newly identified groupings have been dubbed Shakti and Shiva, after two Hindu deities, by their discoverers, Khyati Malhan and Hans-Walter Rix (both at Max Planck Institute for Astronomy, Germany). The astronomers used data gathered by the Gaia astrometry satellite, which has provided spectra and proper motion data for nearly 1.5 billion stars.

Using this data, they were able to tease out two sub-populations of stars with comparable orbital periods and eccentricities. Their metallicities — a rough measure of their ages — shows the two groupings are roughly 11 to 12.5 billion years old, born not long after the Big Bang.

“The metallicity is a birth tag,” Rix tells Sky & Telescope. “It helps preserve a historic record of how pristine or polluted the material was from which they were born.” While the orbits themselves can also suggest a common origin, stars can get kicked around over time, he says.

Ideally, one would like to be able to measure stellar ages to better than 10 percent, which is about the limit of precision when using metallicity as a proxy for age, he says. “The problem is that a lot of stuff happened between 12 and 10 billion years ago, which is still just 11 billion plus or minus 10 percent,” he says.

The combination of the stars’ orbits and composition helps pin down not only their age, but how rapidly these stars became enriched, which serves as an indicator of how densely packed matter was when they formed. “They have signs of being rapidly enriched, yet being quite metal-poor,” Rix says, “which in combination makes us believe the stars were formed fairly early on.”

Two years ago, Rix, Malhan and their team had found “a huge concentration of very similarly old and metal-poor stars at the very center of the Milky Way.” The team called this “the poor old heart of the Milky Way.” Those stars represent one of the first fragments that helped to form our nascent galaxy, the team claimed. The new findings, Rix says, seem to form two separate populations “about halfway between us and the center of the galaxy.”

It may seem surprising that after more than 10 billion years of having merged together, astronomers could still identify the original populations of stars. But Rix compares the process to the observing the small bodies in the solar system. A comet may disintegrate into millions of fragments that gradually spread out along its orbit; however, even when that trail spans millions of miles, it still retains a distinct, clear identity through the similarity of each fragment’s orbit.

“If you plot [the stars] on the sky, it’s hard to see,” he notes. “But if you plot them by eccentricity and size of orbit, then all of a sudden they stick out. And this is what we did.”

This image shows a few different populations of stars, with each color representing stars that joined the Milky Way via a different collision event. Gaia only recently enabled the discovery of Shiva and Shakti (pink and blue, respectively). The left and right frames show two different representations of information about the stars, related to their energy and motions as they move through space.
ESA / Gaia / DPAC / K. Malhan et al. (2024)

Not everyone is convinced, though. Adam Dillamore and others  at the University of Cambridge, UK, have a different interpretation for the stars that Rix and Malhan found. They contend that they cluster due to a resonance effect, perhaps caused by passages of the galaxy’s central bar every several hundred million years. This effect would be similar to, for example, asteroids collecting in resonance with Jupiter’s orbit.

“The primary evidence that these populations are in resonance with the bar comes from their orbital properties, such as their energy,” Dillamore tells Sky & Telescope. “The substructure called Shakti by Malhan & Rix lies at the energy which corresponds to the corotation resonance — when the stars orbit the galaxy with the same average frequency as the bar’s rotation.”

But Dillamore adds that the case is less clear with the other structure, Shiva. “It’s possible that it corresponds to a different resonance, or it may have formed from some other mechanism,” he says. He suggests that simulations of galaxy formation may help to resolve the questions, if similar resonant structures are found in such simulations.

Rix says that in his opinion the resonance interpretation doesn’t quite match the data they see, but the question remains unresolved. “Sometimes you see a striking new phenomenon, and there are just very different explanations for it.”

And that’s just fine, he says: “I think sometimes we need to let people see the soup kitchen of science. It’s not just instant eternal truth that comes out.” With two parallel papers coming out about these findings, “that’s something to be sorted out.”

It all comes down to a basic question, he says: “How much dynamical memory does our Milky Way have?” He compares it to having a city map where you know the exact age and construction materials of every building, so you can start to reconstruct the city’s history over time. “The problem is that spiral arms, bars and all kinds of things eventually move stars from their birth orbits,” Rix says. “So, there’s a dynamical memory loss, and we don’t know how strong it is. I think that’s one of the open questions for the next few years.”

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