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Astronomers have identified two possibilities Milky WayThe earliest building blocks: Named “Shakti” and “Shiva,” they appear to be the remnants of two galaxies that merged with an earlier version of the Milky Way 12 to 13 billion years ago, contributing to the initial development of our home galaxy contribute. The new discovery amounts to archaeologists uncovering traces of an initial settlement that later developed into a large city today. It involves combining data from nearly 6 million stars from ESA’s Gaia mission with measurements from the SDSS sky survey.The results have been published in The Astrophysical Journal.
The early history of our home galaxy, the Milky Way, was composed of smaller galaxies connected together to form sizable building blocks. Now, Kayati Malhan and Hans-Wortrix of the Max Planck Institute for Astronomy have managed to identify two of what are still thought to be the earliest components: fragments of the original Milky Way that merged with an earlier version of our own. . The Milky Way, 12 to 13 billion years ago, was in the earliest stages of galaxy formation in the universe.
Astronomers named the components Shakti and Shiva and identified them by combining data from ESA’s astrometric satellite Gaia with data from the SDSS sky survey. To astronomers, the result amounts to discovering traces of the original settlements that later developed into today’s large cities.
Tracing the origins of stars from other galaxies
When galaxies collide and merge, multiple processes occur simultaneously. Each galaxy carries its own reservoir of hydrogen gas. After the collision, these hydrogen gas clouds became unstable and many new stars formed within them. Of course, the incoming galaxies also already have their own stars, and in the merger, the stars from the galaxies will be mixed together. In the long run, such “accreting stars” will also account for some of the stellar population of newly formed merged galaxies. Once the merger is complete, determining which stars came from which progenitor galaxy seems hopeless. But in fact, there are at least some ways to trace the ancestors of stars.
Help comes from basic physics. When galaxies collide and stellar populations mix, most stars retain very basic properties that are directly related to the speed and direction of the galaxies from which they originated. Stars from the same pre-merger galaxy have similar values in terms of energy and what physicists call angular momentum (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 constant over time. Look for large groups of stars with similar, unusual values of energy and angular momentum – chances are, you might find merger remnants.
Additional instructions may aid in identification. Compared with stars that formed long ago, recently formed stars contain more and heavier elements, which astronomers call “metals.” The lower the metal content (“metallicity”), the earlier stars are likely to form. When trying to identify stars that existed 13 billion years ago, one should look for stars with very low metal content (“metal-poor”).
Virtual Mining in Big Data Sets
Identifying stars that join our Milky Way as part of another galaxy has only recently been possible. It requires large, high-quality data sets, and analysis involves sifting through the data in clever ways to identify the categories of objects being searched for. This data set is only a few years old. ESA’s astrometric satellite Gaia provides an ideal dataset for this big data galactic archeology. Launched in 2013, it has produced increasingly accurate data sets over the past decade, which now include the positions, position changes and distances of nearly 1.5 billion stars in the Milky Way.
Gaia data have revolutionized the study of stellar dynamics in our Galaxy and have led to the discovery of previously unknown substructures. This includes the so-called Gaian Enceladus/Sausage Stream, which is the remnant of the most recent large merger our home galaxy underwent between 8 and 11 billion years ago. It also includes two structures identified in 2022: the Pontic Stream, identified by Malhan and colleagues, and the “poor old heart” of the Milky Way, identified by Rix and colleagues. The latter are newly formed groups of stars during the initial mergers that created the original Milky Way and continue to populate the central region of our galaxy.
Traces of Shakti and Shiva
In the current search, Malhan and Ricks used Gaia data combined with detailed stellar spectra from the Sloan Digital Sky Survey (DR17). The latter provides detailed information about the star’s chemical composition. “We observed that for a range of metal-poor stars, the stars cluster around two specific combinations of energy and angular momentum,” Malhan said.
Contrary to the “poor old heart” also visible in these images, the two groups of like-minded stars have relatively large angular momentum, consistent with a population of stars belonging to a separate galaxy that merged with the Milky Way. Way. Malhan named the two structures Shakti, one of the main deities of Hinduism, and Shiva, the former a female cosmic force often depicted as Shiva’s consort.
Their energy and angular momentum values, coupled with their overall low metallicity comparable to “poor old heart,” make Shakti and Shiva good candidates for our galaxy’s earliest ancestors. “Shakti and Shiva may have been the first two new members of the ‘poor old heart’ of our galaxy to begin its progression into the larger galaxy,” Ricks said.
Several surveys are either already underway or will start within the next few years and are expected to provide relevant additional data, including spectra (SDSS-V, 4MOST) and precise distances (LSST/Rubin Observatory), that should enable astronomers to Make a firm decision as to whether Shakti and Shiva are indeed a glimpse into our galaxy’s earliest prehistory.
Reference: “Shiva and Shakti: Inferred Protogalactic Fragments of the Inner Galaxy,” Khyati Malhan and Hans-Walter Rix, March 21, 2024 The Astrophysical Journal.
DOI: 10.3847/1538-4357/ad1885
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