A group of ancient stars in chaotic orbits suggests that a dwarf galaxy named Loki may have merged with the infant Milky Way galaxy a long time ago.
The Large Magellanic Cloud (LMC) is a dwarf galaxy orbiting our galaxy with about 1% the mass of the Milky Way. A dwarf galaxy named Loki, perhaps twice the size of the LMC, may have merged with the Milky Way early in the galaxy’s history, a new study says. Credit: ESO/VMC Research
The Milky Way Galaxy has had a violent history. It did not grow by simple, gentle accretion, but rather by the active merger of multiple small galaxies. Recently, a team of astronomers discovered the remains of what is likely the oldest merger in the immediate vicinity of our star.
Stars in the Milky Way galaxy vary in age, size, and are rich in metals (elements heavier than hydrogen or helium). Stars born together tend to share many properties because they all originate from the same primordial gas cloud. But even if stars are born together, they quickly disperse as they wander along their respective orbits. Astronomers therefore focus on subtle differences, such as metal abundance, to help identify distinct populations. And through this technique, astronomers can find clusters of stars that weren’t born in our galaxy at all, but got hopped on during violent merging events.
In a recently published paper, arXiv Using a preprint server, a team of astronomers studied an interesting population of 20 stars currently orbiting close to the Sun. But unlike the Sun, these stars have very little metal. This means the star is incredibly old. Because heavy elements result from nuclear fusion inside stars, stars must go through many cycles of life and death for new generations of metals to form.
Although these stars do not contain many metals, they do contain some metals, all in roughly equal amounts, indicating that they share a common origin.
However, half of the stars orbit the Milky Way in one direction, and the other half orbit in the opposite direction.
detective work
With the pieces of the puzzle in place, researchers began the task of making sense of it all. First, these stars are much older than typical stars in the Milky Way disk, indicating that these stars likely did not originate in our galaxy. Simulations of the evolution and distribution of elements within galaxies suggest that these stars most likely originated in small dwarf galaxies, weighing less than about 2 percent of the mass of the present-day Milky Way. Deafness has been suggested.
The universe has many creative ways to produce heavy elements. Massive stars go supernova at the end of their lives, neutron stars merge, and white dwarfs accumulate enough material to go into their own kind of supernova explosion. The particular mix of heavy elements that astronomers found within the star population suggests that the star was enriched by the explosion of a giant star or the merger of a neutron star, rather than an explosion of a white dwarf. This means that the lifespan of the star’s parent galaxy was so short and extreme that there was a lot of star formation (and star death) without enough time for the white dwarf to go supernova. .
Astronomers named the parent galaxy of these invading stars Loki, after the trickster god in Norse mythology. While it’s difficult to pinpoint the exact time when Loki and the Milky Way merged, researchers can look to another clue: the fact that these stars don’t all share the same orientation in their orbits. can.
To explain its strange behavior, Loki merged with the Milky Way very early in its history, long before our galaxy formed its own disk of stars that shared a roughly common orbital direction. I’m sure you did. When the galaxies merged, Loki’s star was scattered in all directions.
Given that the Milky Way galaxy began merging about 10 billion to 11 billion years ago, Loki could potentially be one of the first merging encounters, leaving this strange group of 20 stars behind in the galaxy. This makes it one of the oldest surviving groups.
Through this kind of detective work, astronomers are piecing together not just Loki’s story, but the stories of all the mergers that led to our current galaxy.
