Astronomers have identified two of the Milky Way’s earliest building blocks. The structures called “Shakti” and “Shiva” are likely to be the remnants of two galaxies that merged with an early version of the Milky Way 12 to 13 billion years ago and thus contributed to the early growth of our home galaxy. A press release from the Max Planck Institute for Astronomy (MPIA).
Source: Max Planck Institute for Astronomy March 21, 2024.
March 21, 2024 – The new find is the astronomical equivalent of archaeologists finding traces of an early settlement that later developed into a large city. The reconstruction was achieved using data for almost 6 million stars from ESA’s Gaia mission and the SDSS survey. The results were published in the Astrophysical Journal.
The early history of our home galaxy, the Milky Way, is a story of the joining together of smaller galaxies – quite large building blocks in themselves. Now Khyati Malhan and Hans-Walter Rix from the Max Planck Institute for Astronomy have been able to identify two of the earliest such building blocks that can still be recognized as such today: proto-galactic fragments that occurred 12 to 13 billion years ago, when galaxies were forming Universe was still in its infancy, merging with an early version of our Milky Way. Astronomers have named these components Shakti and Shiva. They were identified by combining data from ESA’s Gaia astrometry satellite with data from the SDSS survey. For astronomy, this find is synonymous with the discovery of traces of the first settlement that developed into a large modern city.
Tracking the origin of stars from other galaxies
When galaxies collide and merge, several things happen in parallel: Each galaxy carries with it its own reservoir of hydrogen gas. During the collision, these hydrogen gas clouds become unstable. Sub-regions of it collapse and form numerous new stars. On the other hand, the incoming galaxies also have their own stars, and during a merger the stars of the galaxies mix. In the long term, these “accreted stars” become part of the stellar population of the newly formed combined galaxy. One might think that it would be hopeless to find out subsequently, namely after the merger process has completed, which stars came from which predecessor galaxy. In fact, there are at least some ways to trace the lineage of such stars.
Fortunately, physics can help at this point. When galaxies collide and their star populations mix, most stars retain fundamental properties that are directly related to the speed and direction of the galaxy from which they originate. Stars from the same galaxy that merged with our Milky Way have very similar values for both their energy and what physicists call angular momentum – in simple terms, the momentum associated with circular motion or rotation. For stars moving in a galaxy’s gravitational field, both energy and angular momentum are conserved over long periods of time. Larger groups of stars, all with roughly the same unusual values of energy and angular momentum, are good candidates for the remnant of a galaxy that merged with the Milky Way.
Further clues can help with identification. Stars that formed more recently contain a higher proportion of heavier elements (in astronomy terms, “metals”) than stars that formed long ago. The lower the metal content (the “metallicity”), the more likely the star in question was formed. If you’re trying to identify stars that existed 13 billion years ago, you should look for stars with very low metal content (“metal poor”).
Virtual data excavations
However, it has only been possible for a relatively short time to identify stars in this way that joined our Milky Way during such a merger process. This requires large, high-quality data sets that must be skillfully sifted for analysis to identify the class of objects being sought – and such data sets have only been available for a few years. ESA’s Gaia astrometry satellite provides an ideal dataset for this type of galactic big data archaeology. Launched in 2013, over the past decade it has produced an increasingly accurate data set that now includes positions, position changes and distances for almost 1.5 billion stars in our galaxy.
The Gaia data have put the study of the dynamics of the stars in our home galaxy on a completely new basis. Based on this data, a whole series of new structures have already been discovered in our Milky Way. One of these is the Gaia-Enceladus/Sausage star stream, a remnant of the most recent major merger that our home galaxy underwent – 8 to 11 billion years ago. Two additional structures were identified in 2022: the Pontus Stream, identified by Malhan and colleagues, and the “poor old heart” of the Milky Way, identified by Rix and colleagues. The latter is a population of stars that were newly formed during the very first mergers that formed the proto-Milky Way and continue to reside in the central region of our galaxy.
Traces of Shakti and Shiva
Malhan and Rix made the discovery described here using Gaia data combined with high-resolution star spectra from the Sloan Digital Sky Survey (DR17). The latter provide detailed information about the chemical composition of stars. Malhan says: “For a particular group of metal-poor stars, there is a clustering at two particular combinations of energy and angular momentum.”
In contrast to the “poor old heart” that the astronomers also identified in the corresponding diagrams, the two groups of like-minded stars had a comparatively large angular momentum. This is exactly what you would expect for groups of stars that belonged to separate galaxies, which then merged with the Milky Way. Malhan called these two structures Shakti and Shiva, the latter one of the main deities of Hinduism and the former a female cosmic force often depicted as the consort of Shiva.
The energy and angular momentum values, as well as the overall low metallicity, roughly equivalent to that of the “poor old heart,” make Shakti and Shiva good candidates for some of the earliest ancestors of our Milky Way. Rix says: “Shakti and Shiva may be the first two new additions to the ‘poor old heart’ of our Milky Way, initiating its growth into a large galaxy.”
Several surveys, either already underway or set to begin in the next few years, promise relevant additional data, both spectra (SDSS-V, 4MOST) and precise distances (LSST/Rubin Observatory). They should allow astronomers to make a clear decision about whether or not Shakti and Shiva are actually a glimpse into the earliest prehistory of our home galaxy.
Original publication
K. Malhan & H.-W. Rix
SHIVA and SHAKTI: Presumed Proto-Galactic Fragments in the Inner Milky Way
The Astrophysical Journal, Vol. 964 (Issue 2), 104 (2024)
dx.doi.org/10.3847/1538-4357/ad1885
pdf: https://www.mpg.de/21722952/malhan_rix_2024_apj_964_104.pdf