The image shows stars identified as belonging to "Shakti" (pink) and "Shiva" (green) in the "Gaia" dataset. Image Source: American Association for the Advancement of Science website.
In the early history of the Milky Way, smaller galaxies merged to form larger structures. Recently, scientists from the Max Planck Institute for Astronomy in Germany have successfully identified two regions that could be among the earliest components of the Milky Way. They are still considered to be remnants of the primitive Milky Way, merging with the early Milky Way about 12 to 13 billion years ago. The research results were published in the recent issue of "The Astrophysical Journal".
Astronomers have named these two components "Shakti" and "Shiva". For astronomers, this finding is akin to discovering traces of the original settlement, which later developed into today's "metropolis".
When galaxies collide and merge, several processes occur simultaneously. Each galaxy carries its own reservoir of hydrogen gas. After the collision, these hydrogen clouds become unstable, forming numerous new stars within. During the merger, stars from the galaxies mix together, making it seemingly impossible to determine which stars came from which progenitor galaxy. However, in basic physics, there are still methods to trace the ancestors of stars.
When galaxies collide and star clusters merge, most stars retain very basic properties directly related to the velocity and direction of their originating galaxies. Stars from the same progenitor galaxy have similar values in their energy and angular momentum. For stars moving in the gravitational field of galaxies, energy and angular momentum are conserved: they remain constant over time. Searching for large groups of stars with similar, unusual energy and angular momentum values likely contains remnants of mergers.
Compared to stars formed a long time ago, recently formed stars contain heavier elements, known as "metals" in astronomy. The lower the metallicity, the earlier the stars may have formed.
In this case, "Shakti" and "Shiva" were identified by combining data from the European Space Agency's Gaia satellite with data from the Sloan Digital Sky Survey (SDSS). Both of these structures have very low metallicity and relatively high angular momentum, consistent with some star clusters belonging to independent galaxies merging with the Milky Way. These factors make "Shakti" and "Shiva" excellent candidates for the earliest "ancestors" of the Milky Way.
