Researchers needed to explain the origin of the massive and rich in "metals" (elements heavier than hydrogen and helium) M31 stellar halo, which contains stars in middle age, as well as giant stellar stream in it.
Astronomers at the University of MI (U-M) announced today (July 23, 2018) that the Andromeda galaxy - closest large spiral galaxy to our Milky Way - shredded and cannibalized another massive galaxy two billion years ago.
Scientists have been aware for some time that a large galaxy like Andromeda probably ate smaller galaxies around it; that's where the halo of stars surrounding Andromeda comes from.
Using computer models, Richard D'Souza and Eric Bell of the University of MI in the U.S. were able to piece together this evidence, revealing this long-lost sibling of the Milky Way.
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"It was a "eureka" moment", D'Souza said.
Using new computer simulations, the scientists were able to understand that even though many companion galaxies were consumed by Andromeda, most of the stars in the Andromeda's outer faint halo were mostly contributed by shredding a single large galaxy. This long-dead galaxy was at least 20 times as massive as any galaxy that has ever merged with the Milky Way.
According to the experts, the discovery could finally help to explain how Andromeda's satellite galaxy M32 formed. The theory is that when Andromeda destroyed M32p, it's center became the unique M32.
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"M32 is a weirdo", Bell said in a statement. It's one of the most compact galaxies in the Universe. "There isn't another galaxy like it".
The new study, which was published online today (July 23) in the journal Nature Astronomy, should help scientists better understand the evolution and effects of galaxy mergers, D'Souza and Bell said. However, there was actually no means to find how many galaxies it had devoured throughout its existence, nor how voracious its appetite might have been.
Massive galaxies such as our own Milky Way have a violent history of drawing in and ripping apart smaller galaxies unfortunate enough to have been captured in their gravitational well. Another research team independently determined earlier this year that Andromeda likely underwent a big merger, and a concomitant surge of star formation, between 1.8 billion and 3 billion years ago. But Andromeda has retained its spiral disk, suggesting that the conventional wisdom does not always hold.
"This work might also solve a long-standing mystery: the formation of Andromeda's enigmatic satellite galaxy", the astronomers said. According to the researchers, the fact that Andromeda's spiral disk managed to survive the impact means that galactic disks may be much more resilient to impacts than astronomers previously thought.
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