The Milky Way is ancient and massive, a collection of hundreds of billions of stars, some dating back to the Universe\u2019s early days. During its long life, it\u2019s grown to these epic proportions through mergers with other, smaller galaxies. These mergers punctuate our galaxy\u2019s history, and its story is written in the streams of stars left behind as evidence after a merger.<\/p>\n
And it\u2019s still happening today. <\/p>\n
<\/p>\n The Milky Way is currently digesting smaller galaxies that have come too close. The Large and Small Magellanic Clouds feel the effects as the Milky Way\u2019s powerful gravity distorts them and siphons a stream of gas and stars from them to our galaxy. A similar thing is happening to the Sagittarius Dwarf Spheroidal Galaxy and globular clusters like Omega Centauri. <\/p>\n There\u2019s a long list of these stellar streams in the Milky Way, though the original galaxies that spawned them are long gone, absorbed by the Milky Way. But the streams still tell the tale of ancient mergers and absorptions. They hold kinematic and chemical clues to the galaxies and clusters they spawned in. <\/p>\n As astronomers get better tools to find and study these streams, they\u2019re realizing the streams could tell them more than just the history of mergers. They\u2019re like strings of pearls, and their shapes and other properties show how gravity has shaped them. But they also reveal something else important: how dark matter has shaped them.<\/p>\n Since dark matter is so mysterious, any chance to learn something about it is a priority. As researchers examine the stellar streams, they\u2019re finding signs of disturbances in them\u2014including missing members\u2014that aren\u2019t explained by the Milky Way\u2019s mass. They suspect that dark matter is the cause. <\/p>\n \u201cIf we find a pearl necklace with a few scattered pearls nearby, we can deduce that something may have come along and broken the string.\u201d<\/p>\n<\/blockquote>\n<\/figure>\n Soon, astronomers will have an enormously powerful tool to study these streams and dark matter\u2019s role in disturbing them: the Vera Rubin Observatory (VRO). <\/p>\n Astronomers have different methods of studying dark matter. Weak gravitational lensing is one of them, and it maps dark matter on the large scale of galaxy clusters. But stellar streams are at the opposite end of the scale. By mapping them and their irregularities and disturbances, astronomers can study dark matter at a much smaller scale.<\/p>\n The Rubin Observatory will complete its Legacy Survey of Space and Time (LSST) in a ten-year period. Alongside its time-domain astronomy objectives, the LSST will also study dark matter. The LSST Dark Energy Science Collaboration is aimed at dark matter and will use Rubin\u2019s power to advance the study of dark energy and dark matter like nothing before it. \u201cLSST will go much further than any of its predecessors in its ability to measure the growth of structure and will provide a stringent test of theories of modi?ed-gravity,\u201d their website explains. <\/p>\n As we get closer and closer to the observatory\u2019s planned first light in January 2025, the growing excitement is palpable.<\/p>\n \u201cI\u2019m really excited about using stellar streams to learn about dark matter,\u201d said Nora Shipp, a postdoctoral fellow at Carnegie Mellon University and co-convener of the Dark Matter Working Group in the Rubin Observatory\/LSST Dark Energy Science Collaboration. \u201cWith Rubin Observatory we\u2019ll be able to use stellar streams to figure out how dark matter is distributed in our galaxy from the largest scales down to very small scales.\u201d<\/p>\n Astronomers have ample evidence that a halo of dark matter envelops the Milky Way. Other galaxies are the same. These dark matter halos extend beyond a galaxy\u2019s visible disk and are considered basic units in the Universe\u2019s large-scale structure. These haloes may also contain sub-haloes, clumps of dark matter bound by gravity. <\/p>\n These clumps are what astronomers think are leaving their marks on stellar streams. The dark matter clumps create kinks and gaps in the streams. The VRO has the power to see these irregularities on a small scale and over a ten-year span. \u201cBy observing stellar streams, we\u2019ll be able to take indirect measurements of the Milky Way\u2019s dark matter clumps down to masses lower than ever before, giving us really good constraints on the particle properties of dark matter,\u201d said Shipp.<\/p>\n The Lambda Cold Dark Matter (Lambda CDM) model is the standard model of Big Bang Cosmology. One of the Lambda CDM\u2019s key predictions says that many sub-galactic dark matter substructures should exist. Astronomers want to test that prediction by observing these structures\u2019 effect on stellar streams. The VRO will help them do that and will also help them find more of them and build a larger data set.<\/p>\n Stellar streams are difficult to detect. Their kinematics give them away, but sometimes, there are only a few dozen stars in the streams. This obscures them among the Milky Way\u2019s myriad stars. But the VRO will change that.<\/p>\n The VRO will detect streams at much further distances. On the outskirts of the Milky Way, the streams have interacted with less matter, making them strong candidates for studying the effect of dark matter in isolation. <\/p>\n \u201cStellar streams are like strings of pearls, whose stars trace the path of the system\u2019s orbit and have a shared history,\u201d said Jaclyn Jensen, a PhD candidate at the University of Victoria. Jensen plans to use Rubin\/LSST data for her research on the progenitors of stellar streams and their role in forming the Milky Way. \u201cUsing properties of these stars, we can determine information about their origins and what kind of interactions the stream may have experienced. If we find a pearl necklace with a few scattered pearls nearby, we can deduce that something may have come along and broken the string.\u201d<\/p>\n The VRO\u2019s powerful digital camera and its system of filters make this possible. Its ultraviolet filter, in particular, will help make more streams visible. Astronomers can distinguish stellar streams from all other stars by examining the blue-ultraviolet light at the end of the visible spectrum. They\u2019ll have thousands upon thousands of images to work with. <\/p>\n In fact, the VRO will unleash a deluge of astronomical data that scientists and institutions have been preparing to handle. AI and machine learning will play a foundational role in managing all that data, which should contribute to finding even more stellar streams. <\/p>\n \u201cRight now it\u2019s a labor-intensive process to pick out potential streams by eye\u2014Rubin\u2019s large volume of data presents an exciting opportunity to think of new, more automated ways to identify streams.\u201d<\/p>\n Astronomers are still finding more stellar streams. Earlier this month, a paper in The Astrophysical Journal presented the discovery of another one. Researchers found it in Gaia\u2019s Data Release 3. It\u2019s likely associated with the merger of the Sequoia dwarf galaxy.<\/p>\n It seems certain that astronomers will keep finding more stellar streams. Their value as tracers of the Milky Way\u2019s history is considerable. But if scientists can use them to understand the distribution of dark matter on a small scale, they\u2019ll get more than they bargained for. <\/p>\n\n
![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/04\/sgrstream-768x577-1.jpeg\")
![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/04\/CDM-halo-and-sub-haloes.jpg\")
\ngenerations of sub-subhaloes contained within subhalo f. Image Credit: Zavala and Frenk 2019<\/figcaption><\/figure>\n![\"Rubin](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2023\/06\/Rubin_May_2022.jpg\")
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