Over the course of the history of the Universe, the Milky Way has not been sailing smoothly. Quite the opposite. Over the last 13 billion years or so, it has collided with and eaten other galaxies.
Astronomers led by Khyati Malhan of the Max Planck Institute for Astronomy in Germany have mapped six of these ancient galactic mergers, five of which were already known. The results will lead to a better understanding of the history, growth and evolution of the stars.
The researchers wrote in their paper that the dynamical atlas of the Milky Way mergers provides a global view of the galaxy formation in action.
Our study contributes to the initial steps of unraveling the full hierarchical build-up of our galaxy and also understanding the origin of the globular clusters and stellar streams of the Milky Way halo.
There is more to the universe than a disk of stars around a black hole. The orb encircles the disk and its surrounds, and extends above and below the plane. The structure is called the galactic halo, and it was formed mostly from mergers with other galaxies.
It takes innovative detective work to work out that history. It is difficult to know how far away things are in space. Unless you know how bright a star is, it's hard to figure out its distance. This made it difficult for groups of stars.
The results are not a neat subsumption. We call it a stellar stream because the tidal forces stretch the other galaxy out so that it forms a river of stars.
There are also dense clusters of stars called globular clusters and satellite galaxies that are thought to be remnants of galactic mergers.
S. Payne-Wardenaar and K. Malhan are members of the MPIA.
Astronomers have found more of these streams thanks to a project called Gaia. The discoveries it has enabled are pretty spectacular, including the Gaia-Enceladus merger that took place 9 billion years ago, and the fact that the position and motion of stars in three-dimensional space is the highest accuracy yet.
Malhan and his colleagues used a statistical procedure to identify whether or not the groups were linked to a galactic merger after the early third release of Gaia data. The team included 170 globular clusters, 41 stellar streams, and 46 satellite galaxies, and assigned 62 of them to six merger events with smaller galaxies.
There were five that were known. The Cetus merger, discovered in 2020, was one of several that happened in the last 9 billion years.
S. Payne-Wardenaar and K. Malhan are members of the MPIA.
The team discovered a new merger called Pontus. The stars associated with Pontus are moving very slowly against the main rotation of the Milky Way. The merger could have taken place as early as 8 to 10 billion years ago, they believe.
Some of the oldest stars in the Milky Way are found in three of the streams associated with LMS-1/Wukong. It is possible that the progenitor galaxy of those stars may have formed very early in the history of the Universe.
There are also hints of other mergers in the sky. The automated system missed two known mergers, including the Kraken, and a manual analysis of the data by the team suggested that there was yet another unknown merger. Some of the 195 remaining objects could be tied to these, or they could be from smaller mergers that left behind smaller traces.
The next step is to reconstruct a timeline of all the mergers.
They write that they will be able to explore the archeology aspect of the Milky Way by building an understanding of the merging history.
The research has been published.
All Rights Reserved © 2024
