Before you continue thinking about our Milky Way galaxy as a peaceful, spiral disk of stars slowly rotating through the heavens, consider this: It is hungrily digesting the remains of another galaxy trapped in its grip, all the while flapping its outer edge.
It is hardly neighborly to eat a visitor, but that is exactly what the Milky Way appears to be doing.
Astronomers have concluded this after mapping the stars in half of the northern sky with the Sloan Telescope in New Mexico. The project is measuring distances to nearly 50-million stars, building a three-dimensional map of the Milky Way.
In the process, scientists have noticed a huge, very faint group of hundreds of thousands of seemingly alien stars spread out over an area about 5,000 times the size of a full moon. This star group is within our galaxy's confines, about 30,000 light years away, the distance light travels in that length of time.
The collection does not seem to be part of the galaxy's central bulge, nor of the flattened disk of stars around the bulge where our solar system resides, nor of the extended, spherical halo of stars surrounding the galaxy.
Princeton University astronomer Robert Lupton says the most likely interpretation of the new structure is of another, smaller galaxy being consumed by the Milky Way.
"We're seeing a cloud of stars somewhere stuck above the plane of our galaxy. We believe it is almost certainly a dwarf galaxy merging with the Milky Way. It is almost certainly a galaxy being ripped up, eaten, and otherwise digested by our galaxy," said Lupton. "So the Milky Way is still growing. It's not a static system. It's growing by cannibalizing smaller neighbors."
Lupton says astronomers have come to understand that such mergers are the way galaxies expand.
"People would imagine that there was, long ago, a period when many galaxies grew by mergers," he said. "If you look out at the well known galaxies that we see around us, many of those are still interacting, although interacting was very much commoner for big galaxies in the past."
The merger of the Milky Way with the dwarf galaxy creates an overdensity of stars in that region of the Milky Way, which covers an area 45,000 light years across. Lupton told the American Astronomical Society meeting in Washington that our galaxy is taking a long time to swallow its dwarf neighbor.
"It's being eaten and it's going to be part of the Milky Way in another billion years," added Lupton. "So welcome to our newest member."
Meanwhile, another pair of satellite galaxies is causing the Milky Way to warp at its edge, much like the upturn in the brim of a hat.
Scientists have long known about the warpage, but by studying patterns of hydrogen gas, only now do they understand that the cause is the motion of two small nearby galaxies orbiting the Milky Way called the Magellanic Clouds. As these galaxies circle us, they plow through unseen dark matter thought to exist around the galaxy, amounting to an estimated 90 percent of the matter in it. The motion of the Magellanic Clouds through the dark matter creates a wake like a boat sailing through water. This wake increases the dark matter's gravitational pull on the Milky Way's disk, according to astronomer Leo Blitz of the University of California at Berkeley.
"As the Magellanic Clouds orbit the Milky Way, the warp looks like it's flapping in the breeze," he said. "We talk about it flapping like a tablecloth in the breeze. The center doesn't move very much, but the outer parts flap around."
Some astronomers do not believe dark matter is causing the galactic warp or the flapping, but Blitz says the Magellanic Clouds alone are not massive enough to have such a strong effect. Because many other galaxies have warped disks, he says similar dynamics might explain them as well.
"We need dark matter to see the strong bending modes that we see and this is probably a common feature of spiral galaxies in general," added Blitz.