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, writes an Iowa State University astronomer in the journal Nature.

Curtis Struck, an Iowa State professor of physics and astronomy, uses a News and Views commentary in the Sept. 15 issue of Nature to add context and color to a study published in the same issue by a research team led by Chris W. Purcell of the University of Pittsburgh.

The Purcell group reports that 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.

The Purcell group writes that 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.

Struck has studied galaxies for 30 years and focuses his research on galactic collisions. His book, "Galaxy Collisions: Forging New Worlds from Cosmic Crashes," was published earlier this year. (Struck will discuss the book at Iowa State's Parks Library at 3 p.m. on Wednesday, Oct. 26.)

In his Nature commentary, Struck said the Sagittarius paper gives astronomers new ideas to study. Among them: 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.

Struck reported there are several clues to those widespread effects: 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.

Struck also writes that the estimated range of the size of Sagittarius in its original form is likely to be the subject of study and contention. He notes the estimates at the smaller end of the range indicate the dwarf galaxy would have smaller, but still significant, impacts on the Milky Way.

That means a galactic mouse could have once been a bear that's "been messing things up for some time," Struck wrote. And so, he warned, "Beware of the wildlife, even in apparently quiet galaxies."

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