Visualization of the Milky Way, with the stars identified by Khyati Malhan and Hans-Walter Rex in the Gaia DR3 dataset as belonging to Shiva and Shakti, also shown as colored dots. Shiva's stars are shown in green and Shakti's stars are shown in pink. The complete absence of green and pink signs in some regions does not mean that there are no Shiva or Shakti stars there, as the data set used in this study only covers specific regions within our galaxy. Credit: S. Payne-Wardenar / K. Malhan / MPIA
Astronomers have identified what two of these planets could be milky wayThe oldest building blocks of Shakti and Shiva appear to be the remains of two galaxies that merged 12 to 13 billion years ago with an early version of the Milky Way, contributing to the initial growth of our home galaxy. The new discovery is the astronomical equivalent of archaeologists identifying traces of an initial settlement that grew into a large city today. It required combining data on nearly 6 million stars from the European Space Agency's Gaia mission with measurements from the SDSS survey. The results have been published in Astrophysical Journal.
The early history of our parent galaxy, the Milky Way, is one of joining up with smaller galaxies, resulting in rather large building blocks. Now, Khyati Malhan and Hans-Walter Rex of the Max Planck Institute for Astronomy have succeeded in identifying what could be some of the oldest building blocks that can still be recognized as such today: fragments of a protogalaxy that merged with an early version of our galaxy. The Milky Way occurred 12 to 13 billion years ago, at the beginning of the era of galaxy formation in the universe.
The components, which astronomers have named Shakti and Shiva, were identified by combining data from the European Space Agency's Gaia astrometry satellite with data from the SDSS survey. For astronomers, the result is equivalent to finding traces of an initial settlement that developed into a large city today.
Tracing the origins of stars that came from other galaxies
When galaxies collide and merge, several processes occur in parallel. Each galaxy holds its own reservoir of hydrogen gas. Upon impact, these clouds of hydrogen gas are destabilized, and many new stars are formed inside them. Of course, the upcoming galaxies already have their own stars, and in the merger process, the stars from the galaxies will mix. In the long term, such “accreting stars” will also form some of the star clusters of the newly merged galaxy. Once the merger process is complete, it may seem pointless to determine which stars came from which previous galaxy. But in reality, there are at least some ways to trace your stellar lineage.
Help comes from basic physics. When galaxies collide and their star clusters mix, most stars retain very fundamental properties, which are directly related to the speed and direction of the galaxy in which they originated. Stars from the same galaxy before the merger share similar values for both their energy and what physicists call angular momentum — the momentum associated with orbital motion, or rotation. For stars moving in the gravitational field of a galaxy, both energy and angular momentum are conserved: they remain the same over time. Look for large clusters of stars with similar and unusual values of energy and angular momentum – and chances are you might find fusion remnants.
Additional indicators can aid in identification. Stars that formed more recently contain heavier elements, what astronomers call “metals,” than stars that formed long ago. The lower the metal content (“metallicity”), the earlier the star formed. When trying to identify stars that actually existed 13 billion years ago, one should look for stars with very low metal content (“metal poor”).
Virtual fossils in a large dataset
Identifying stars that have joined our Milky Way as parts of another galaxy has only become possible relatively recently. It requires large, high-quality data sets, and the analysis involves sifting through the data in intelligent ways to determine the class of objects being investigated. This type of data set has only been available for a few years. ESA's Gaia astrometry satellite provides an ideal dataset for this type of big data for galactic archaeology. Launched in 2013, it has produced an increasingly accurate data set over the past decade, which now includes the positions, changes in position and distances of nearly 1.5 billion stars within our galaxy.
Gaia data has revolutionized studies of stellar dynamics in our Galaxy, and has already led to the discovery of previously unknown substructures. This includes the so-called Gaia Enceladus/Sausage Stream, a remnant of our galaxy's most recent, largest merger, between 8 and 11 billion years ago. It also includes two structures identified in 2022: the Pontus Stream identified by Malhan and colleagues, and the “poor old core” of the Milky Way identified by Rex and colleagues. The latter is a group of stars that were newly formed during the initial mergers that created the primordial Milky Way, and continue to reside in the central region of our galaxy.
Effects of Shakti and Shiva
In their current research, Malhan and Rex used Gaia data along with detailed stellar spectra from the Sloan Digital Sky Survey (DR17). The latter provides detailed information about the chemical composition of stars. “We observed that for a particular population of metal-poor stars, the stars were crowded around two specific combinations of energy and angular momentum,” says Malhan.
In contrast to the “poor old heart,” which was also visible in those diagrams, the two like-minded star clusters had relatively large angular momentum, consistent with star clusters that were part of separate galaxies that had merged with the Milky Way. road. Malhan named these two structures Shakti and Shiva, the latter being one of the major deities in Hinduism and the former a feminine cosmic force often depicted as Shiva's consort.
Their values of energy and angular momentum, as well as their generally low metallicity on par with those of the “poor old core”, make Shakti and Shiva good candidates for some of the early ancestors of our Milky Way Galaxy. “Shakti and Shiva may be the first additions to the 'poor old heart' of our Milky Way, causing it to start growing into a large galaxy,” says Rex.
Several surveys already underway or scheduled to begin over the next two years promise additional relevant data, both spectra (SDSS-V, 4MOST) and precise distances (LSST/Rubin Observatory), that will enable astronomers to make a firm decision about what… Whether Shakti and Shiva actually represent a glimpse into the earliest prehistory of our home galaxy remains to be seen.
Reference: “Shiva and Shakti: Presumed Protogalactic Fragments in the Inner Milky Way” by Khayati Malhan and Hans-Walter Rex, 21 March 2024, Astrophysical Journal.
doi: 10.3847/1538-4357/ad1885
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