Milky way “vibrating like a drum”
and World Science staff
Our Milky Way galaxy is warped, and vibrates like a drum, because of the influence of two small companion galaxies, astronomers have found.
The researchers said the effect is due to the most prominent of the Milky Way’s satellite galaxies, a pair of galaxies called the Magellanic Clouds. They are stirring up with the Milky Way’s “dark matter,” an invisible substance that’s detectable only by its gravitational pull, the astronomers added. Dark matter is thought to make up more then 90 percent of the weight of the universe.
The interaction creates a warp in the galaxy that has puzzled astronomers for half a century, the reseearchers continued. The warp, most obvious in the thin disk of hydrogen gas permeating the galaxy, extends across the Milky Way’s 200,000-light year width. A light year is the distance light travels in a year.
Leo Blitz, professor of astronomy at the University of California, Berkeley, and colleagues charted this warp and analyzed it in detail for the first time, based on a new galactic map of light given off by the hydrogen gas.
They found the gas layer is vibrating like a drumhead, and that the vibration consists almost wholly of three “notes,” also called modes. These notes would be unimaginably deep by human standards—some three million octaves, or scales, below the note called middle C on a piano.
This means that if a piano could play these notes, it would require a keyboard about the width of Iceland to do so.
It’s not uncommon for astronomical objects to exhibit some sort of regular vibrations, like musical instruments, so that they can be said to be playing notes. Which note depends on the vibration speed. A study last summer found that a violent quake on the surface of a compact type of star called a neutron star left it playing the note of “F sharp.”
Although the Milky Ways’ warp has been known for almost 50 years, astronomers previously dismissed the Magellanic Clouds—comprised of the Large and Small Magellanic Clouds—as its cause because the galaxies’ combined masses are only 2 percent that of Milky Way, Blitz and colleagues said.
This mass was thought too small to influence a massive disk equivalent to about 200 billion suns during the clouds’ 1.5 billion-year orbit of the galaxy.
But Martin D. Weinberg of the University of Massachusetts, Amherst, joined Blitz to create a computer model that takes into account the Milky Way’s dark matter. The motion of the clouds through the dark matter creates a wake that enhances their gravitational influence on the disk, the astronomers found. The wake stirs a vibration at the center of the dark matter blob pervading the galaxy. This in turn makes the embedded galactic disc oscillate.
When this dark matter is included, the Magellanic Clouds, in their orbit around the Milky Way, closely reproduce the type of warp observed in the galaxy, said Blitz, director of UC Berkeley’s Radio Astronomy Laboratory. “The model not only produces this warp in the Milky Way, but during the rotation cycle of the Magellanic Clouds around the galaxy, it looks like the Milky Way is flapping in the breeze.”
“People have been trying to look at what creates this warp for a very long time,” Weinberg said. “Our simulation is still not a perfect fit, but it has a lot of the character of the actual data.” The researchers presented the findings this week at the American Astronomical Society meeting in Washington, D.C.
The Magellanic Clouds’ effect on the dark matter is reminiscent of the paradox that led to dark matter’s discovery some 35 years ago, the researchers said. Astronomers realized stars in the galaxy’s outer regions had speeds much greater than the gravitational pull of the galaxy’s visible stars could account for. Only by assuming most the galaxy’s mass was too dark to see, could astronomers reconcile the velocities with the laws of gravity.
Astronomers now take dark matter into account in computer simulations of cosmic events, even though they don’t know what it is.
Some physicists have devised an alternative theory of gravity called Modified Newtonian Dynamics to explain these observations without resorting to dark matter. But Weinberg said his findings are bad news for that theory, because it can’t easily explain the Milky Way’s warp, whereas dark matter can.
Many galaxies have warps, possibly for similar reasons, he added.
His team mathematically described our galaxy’s vibrations as a combination of three motions: a flapping of the disk’s edge up and down, an up-and-down motion like that of a drumhead, and a saddle-shaped oscillation.
That it took just three “notes” to describe the motion was “very surprising,” Blitz said, noting that this had escaped astronomers’ notice since they discovered the warp discovery in 1957. “this simple, elegant vibrational structure just popped out.”
The findings are to be published in an upcoming issue of the research journal Astrophysical Journal.
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