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The uncertainty of science: Astronomers have now roughly mapped out the history of the Andromeda galaxy, identifying two major events whereby it had absorbed nearby dwarf galaxies.
“It’s been known for 10 to 15 years that Andromeda has a vigorous history of accumulating and destroying its neighbours,” Mackey says. In fact, he says, “It seems to have a much more intense history of that than the Milky Way.”
…[New data] “tells us there were two main events that formed the halo of Andromeda,” Mackey says. “One occurred very long ago. The other must have happened relatively recently.”
Not that Andromeda couldn’t also have eaten innumerable smaller galaxies. ”We can’t trace them with galactic clusters, because they didn’t have any to begin with,” Mackey says.
Most of the news reports about this new research have been very overwrought (Andromeda is “violent” and is going to “eat us!”) and very unaware that the assimilation of nearby small galaxies by Andromeda is not really news. Astronomers have known for years that big galaxies like Andromeda and the Milky Way absorb the dwarf galaxies around them. All this story does is postulate a more detailed though very rough timeline.