Even though a dwarf galaxy clear across the Milky Way looks to be a mouse, it may have once been a bear that slashed through the Milky Way and created the galaxy's spiral arms. What does all this mean?
Astronomer Curtis Struck thinks the Sagittarius Dwarf Elliptical Galaxy collided with the Milky Way, creating the galaxy’s spiral arms, its central bar structure and the flaring at its outer disk. Along the way, the dwarf galaxy’s stars were scattered and the galaxy shrunk to an object that's so small and unimpressive it's hard to see. Maybe the evolution of our Milky Way galaxy did include collisions and wasn’t as peaceful as astronomers had thought.
And, even though these dwarf galaxies are dim and difficult to detect, the effects of their collisions with bigger galaxies may be widespread across the universe. Dwarf galaxies such as the Sagittarius originally had massive amounts of dark matter. The original galaxy, in other words, may have been 100,000 times more massive than it is today.
And that may have given it the power to shape our galaxy. In recent years several dozen of the dwarf galaxies have been discovered around the Milky Way and Andromeda galaxies. The effects of galaxy collisions with these dwarfs could be long lived. And models of galaxy formation indicate that these dwarf galaxies could be common and important parts of galaxy growth.
Sturck says this means a galactic mouse could have once been a bear that's "been messing things up for some time. Beware of the wildlife, even in apparently quiet galaxies."
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