Ancient structures that helped form the Milky Way are identified

Astronomers used the Gaia space telescope to observe some of the Milky Way galaxy's earliest building blocks: two ancient streams of stars called Shakti and Shiva, which helped our galaxy grow and evolve more than 12 billion years ago.

The stellar streams, named after Hindu deities, appear to be the remnants of two galaxies that merged with an early version of the Milky Way between 12 billion and 13 billion years ago, when the first galaxies were forming across the cosmos. The structures are so old that they formed well before the oldest and most iconic parts of the Milky Way's spiral arms and central disk.

A study detailing the observations was published on Thursday (21) in The Astrophysical Journal.

“What's really amazing is that we can detect these ancient structures anyway,” said study lead author Dr. Khyati Malhan, postdoctoral fellow and Humboldt Research Fellow at Stockholm University in Sweden, in a statement. . “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 Gaia has made it possible.”

Observing the stellar wonders of Shakti and Shiva could help astronomers unlock the secrets of the early days of the Milky Way galaxy and the evolution of equally massive galaxies across the cosmos according to the researchers.

First steps in the growth of the Milky Way

The Gaia space telescope, launched in 2013 by the European Space Agency (ESA), began observing the universe the following year. Astronomers have used Gaia observations to discover previously unknown structures in the Milky Way, helping them piece together the history of the galaxy. The telescope's dataset also provided astronomers with the positions, distances and motions of nearly 2 billion stars in the galaxy.

In 2022, study co-author Hans-Walter Rix and his colleagues used Gaia to peer into the heart of the Milky Way and discovered the oldest stars ever found in the galaxy during their “galactic archeology” observations. An analysis of data from nearly 6 million stars observed by Gaia and the Sloan Digital Sky Survey revealed two streams that seemed to stand out from the rest.

You survey data contained details about the chemical composition of stars .

“We observed that for a certain range of metal-poor stars, the stars were clustered around two specific combinations of energy and angular momentum,” said Malhan.

Shakti and Shiva are close to the heart of the Milky Way , and each stream is estimated to include the mass of about 10 million suns. Here, the ancient stars are all similar in age, orbit and composition, which helped astronomers determine that both streams were likely fragments of an external source that became entwined and became part of the Milky Way.

Researchers compared the discovery of Shiva and Shakti to finding the initial traces of an ancient settlement that eventually grew to become a large modern city.

“The stars there are so old that they are missing many of the heavier metallic elements created later in the life of the Universe. These heavy metals are those forged inside stars and spread throughout space when they die. The stars at the heart of our galaxy are metal-poor, so we nicknamed this region the 'poor old heart of the Milky Way,'” said Rix, director of the department of galaxies and cosmology at the Max Planck Institute for Astronomy in Germany, in a statement.

“Until now, we had only recognized these very ancient fragments that came together to form the ancient heart of the Milky Way. With Shakti and Shiva, we now see the first pieces that appear comparatively ancient but located further away. These signify the first steps in the growth of our galaxy towards its current size.”

Tracking galactic evolution

The Milky Way started out small and grew in size as it merged with other galaxies, gaining stars as well as hydrogen to form more stars. Every galaxy has hydrogen gas that helps star birth. As galaxies merge and collide, their hydrogen gas clouds are disturbed, which can create a frenzy of star birth.

Over time, the Milky Way's long filaments of gas and dust came together and resulted in the modern spiral structure of the galaxy today.

Gaia has already helped astronomers determine when the Milky Way experienced different merger events in the past, and future observations could unlock more insights.

“Revealing more about our galaxy's infancy is one of Gaia's goals, and it is certainly achieving that,” said Timo Prusti, project scientist for Gaia at the European Space Agency, who was not involved in the study.

“We need to identify the subtle but crucial differences between stars in the Milky Way to understand how our galaxy formed and evolved. This requires incredibly accurate data – and now, thanks to Gaia, we have that data. As we discover surprising parts of our galaxy, like the Shiva and Shakti currents, we are filling in the gaps and painting a more complete picture of not only our current home, but also our earliest cosmic history.”