Our Milky Way Galaxy Vibrates Like a Drum

According to astronomers, our Milky Way galaxy is warped and vibrates like a drum because of the influence of two companion galaxies. Astronomers say peculiar drum-like vibrations in our Milky Way galaxy may explain why it is warped, and similar explanations may apply for other warped galaxies. 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 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 warp, most obvious in the thin disk of hydrogen gas permeating the galaxy, extends across the Milky Ways 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 Way's warp has been known for almost 50 years, astronomers previously dismissed the Magellanic Clouds as its cause because the galaxies combined masses are only 2 percent that of Milky Way. 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 joined Blitz to create a computer model that takes into account the Milky Ways dark matter. The motion of the clouds through the dark matter creates a wake that enhances their gravitational influence on the disk. 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. 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.