In the realm of modern astronomy, a prevailing theory suggests that major galaxies, including our own Milky Way, have formed through the accumulation of countless smaller mergers. Consequently, it's reasonable to assume that some of the globular clusters within our galaxy might have originated in other galaxies that were absorbed by the Milky Way, or even taken from intact companion galaxies like the Magellanic Clouds.
The connection between these clusters and their diverse origins began to emerge in the 1990s, but recent research is shedding more light on the specific percentage of globular clusters that were "stolen" and which ones exactly.
Historically, the Milky Way has played host to numerous satellite galaxies. These include well-known ones like the Magellanic clouds, but also lesser-known satellites such as the Fornax Dwarf and the Antlia II Dwarf.
However, not all of these dwarf galaxies exist independently. Some, like the Sagittarius Dwarf Galaxy, which was discovered in 1994, exhibit notable elongation and distortion, resembling long streams of stars rather than organized galaxies. This suggests that they are currently being torn apart by the Milky Way's gravitational forces and are in the process of merging with our galaxy.
The discovery of these streams of tidally disrupted systems provided a potential explanation for an intriguing observation: many of the globular clusters surrounding our galaxy share a similar age, while others are relatively young. Astronomers began to theorize that these younger clusters might have formed in these relatively young dwarf galaxies.
Over time, evidence supporting this hypothesis has accumulated. For instance, in 2002, astronomers studying the globular cluster NGC 5634 found that its position coincided with the tidal stream of the Sagittarius Dwarf Galaxy. Its motion and metal-poor composition matched those of the dwarf galaxy.
Since then, astronomers have found convincing evidence linking several additional globular clusters to this tidally disrupted galaxy. These clusters include AM 4, Arp 2, Pal 12, NGC 2419, NGC 4147, Terzan 7, Terzan 8, and Whiting 1.
Furthermore, other streams of dwarf galaxies undergoing tidal disruption have been discovered, such as the Helmi stream, the Gaia-Enceladus Sausage, and the Sequoia galaxy. Subsequently, associations with more globular clusters have come to light.
In addition to galaxies currently being cannibalized, astronomers have also posited that some of the more intact dwarf galaxies orbiting the Milky Way might have contributed globular clusters to our galaxy and may have, in turn, "borrowed" some clusters.
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