The Milky Way's Strange Galactic Neighbors

Fly-bys, hobbit galaxies, and an impending merger with Andromeda

By Jennifer Barone|Tuesday, September 18, 2007

Astronomers are radically reshaping our picture of the Milky Way’s neighbors. Our corner of the cosmos, known as the Local Group, includes two giant spiral galaxies—the Milky Way and Andromeda—and smaller satellite galaxies orbiting them. The Milky Way was thought to have about 10 satellites, but within the last year or so, that number has nearly doubled. “Most astronomers, myself included, thought we at least knew the members of the Local Group,” says Daniel Zucker of Cambridge University, whose team found the new batch of eight galaxies. “I don’t think anyone expected us to find a significant population of these things. They’re fainter than anybody thought a galaxy could be—even smaller and less luminous than what are typically considered dwarf galaxies,” he adds, so they became “hobbit galaxies.” One, Leo T, still has gas associated with it, providing the raw material for stellar births. “It’s arguably the smallest star-forming galaxy known,” Zucker says. The survey that found the satellite galaxies scanned only a fifth of the sky, so there could be dozens more waiting to be found.

In another surprise, Harvard University astrophysicist Nitya Kallivayalil recently announced that two of our largest satellite galaxies probably aren’t satellites at all. Kallivayalil found that the Large and Small Magellanic clouds are shooting by us at around 200 miles per second, faster than a satellite would. With that kind of speed, she says, they are likely to be travelers speeding through the region. The only scenario in which the clouds could remain satellites enthralled in the gravitational pull of the Milky Way requires that the galaxy have twice its currently estimated mass.

Astronomers are also resizing our largest neighboring galaxy, Andromeda, finding that its radius is up to five times larger than anyone thought. Observations revealed a halo of stars half a million light-years from the galaxy’s center but still bound to it. “Much of the space between the Milky Way and Andromeda is filled with stars that belong to those galaxies,” says University of California at Santa Cruz astronomer Raja Guhathakurta, whose team discovered the halo. “They practically overlap. It really challenges the notion of galaxies as groups of stars with empty space between them.”

Future astronomers will become intimately acquainted with Andromeda as it screams toward us on a collision course with the Milky Way. A new simulation indicates that the first pass of galactic jousting will occur in 2 billion years, and the galaxies will fuse within 5 billion years. As the universe expands, all other galaxies will fade from sight. Harvard astrophysicist Avi Loeb, who developed the simulation, says astronomers should appreciate that we live at a special time in cosmic history: “Now when we look up, we see many galaxies. In the distant future, this will become a lonely place, with nothing to look at. If we want to learn about the universe at large, we’d better do it while we still can.”

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