The Milky Way galaxy is composed of many smaller galaxies captured over at least 10 billion years. Although we can detect traces of the more recent galactic mergers in the movement of stars, earlier ones are more obscure. An analysis of globular clusters not only reveals more details about previously-known mergers, but sheds light on an event so mysterious it's known as the Kraken.
When one galaxy captures another tight bundles of stars often maintain a partial independence, becoming globular clusters that orbit the combined object, far enough from the main disk to remain largely unchanged. Dr Diederik Kruijssen of the University of Heidelberg has made a detailed study of the Milky Way’s globular clusters to find common features identifying some as having had a shared origin. Using this information Kruijssen has tried to “reverse engineer” our galaxy’s development.
"The main challenge of connecting the properties of globular clusters to the merger history of their host galaxy has always been that galaxy assembly is an extremely messy process, during which the orbits of the globular clusters are completely reshuffled," Kruijssen said in a statement.
Difficult as this is, Kruijssen and co-authors think they have cracked the code, publishing what they call a “Family Tree” for the Milky Way in Monthly Notices of the Royal Astronomical Society. Based on their ages, orbital paths and metal content the globular clusters are identified as falling into five groups, besides those thought to have formed with the Milky Way initially. Four of these clusters of clusters match the known progenitor galaxies Gaia-Enceladus, Helmi, Sequioa and Sagittarius. The last group of globular clusters, the authors write; “Provide an excellent match to the predicted properties of the enigmatic galaxy ‘Kraken’”. Although the authors think the Kraken galaxy was similar in size to Gaia-Enceladus, they say its absorption had a much bigger impact on the Milky Way.
"The collision with Kraken must have been the most significant merger the Milky Way ever experienced," Kruijssen said. "Before, it was thought that a collision with the Gaia-Enceladus-Sausage galaxy, which took place some 9 billion years ago, was the biggest collision event. However, the merger with Kraken took place 11 billion years ago, when the Milky Way was four times less massive. As a result, the collision with Kraken must have truly transformed what the Milky Way looked like at the time."
The five proto-galaxies described each had masses at least 60 million times that of the Sun. The authors think there were at least ten galaxies captured with more than 4.5 million solar masses. There were probably others small enough people might debate whether they deserve the name galaxy.
Nevertheless, important as these events were to the growth of our galaxy, Kruijssen and co-authors conclude most of the stars formed “in-situ”. Most stars in the galaxy come either from the Milky Way's original composition, or from gas that condensed enough to start fusion after being captured.