The LMC appears in the Southern Hemisphere's night sky. In this photo illustration, the image contrast in a ground-based photo was enhanced to highlight the LMC’s faint outer regions, which are not visible to the naked eye.
To illustrate the LMC's large apparent size on the sky, an image of the full moon is shown at bottom right. A horizon has been added for perspective.
The arrows represent the highest-quality Hubble measurements of the motion of the LMC's stars to show how this galaxy rotates.
Each arrow reveals the predicted motion over the next 7 million years. The motion of each star measured by Hubble over a few years’ time is a million times smaller than the length of each arrow.
The LMC completes a rotation every 250 million years.
Credit: NASA, ESA, A. Feild and Z. Levay (STScI), Y. Beletsky (Las Campanas Observatory), and R. van der Marel (STScI)
Using the NASA /ESA Hubble Space Telescope, astronomers have for the first time precisely measured the rotation rate of a galaxy based on the clock-like movement of its stars.
According to their analysis, the central part of the neighboring galaxy, called the Large Magellanic Cloud (LMC), completes a rotation every 250 million years.
Coincidentally, it takes our Sun the same amount of time to complete a rotation around the center of our Milky Way galaxy.
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| Roeland van der Marel |
Hubble recorded the stars' slight movements over a seven-year period.
Disk-shaped spiral galaxies, like the Milky Way and the LMC, generally rotate like a carousel.
Hubble's precision tracking offers a new way to determine a galaxy's rotation by the "sideways" proper motion of its stars, as seen in the plane of sky.
Astronomers have long measured the sideways motions of nearby celestial objects, but this is the first time that the precision has become sufficient to see another distant galaxy rotate.
For the past century astronomers have calculated galaxy rotation rates by observing a slight shift in the spectrum, called the Doppler effect, of its starlight.
On one side of a galaxy's spinning stellar disk, the stars swinging in the direction of Earth will show a spectral blueshift (the compression of light waves due to motion toward the observer).
Stars swinging away from Earth on the opposite side of a galaxy will show a spectral redshift (the stretching of light to redder wavelengths due to motion away from the observer).
The newly measured Hubble sideways motions and the Doppler motions measured previously each provide complementary information about the LMC's rotation rate.
By combining the results, the Hubble team for the first time obtained a fully three-dimensional view of stellar motions in another galaxy.
This animation illustrates the rotation rate of the Large Magellanic Cloud (LMC).
Hubble Space Telescope observations have determined that the central part of the LMC completes a rotation every 250 million years.
Hence, it takes more than 10 million years for even the small amount of rotation illustrated here.
Credit: NASA, ESA, and G. Bacon, R. van der Marel, A. Feild, L. Frattare, Z. Levay, and F. Summers (STScI)
"Determining a galaxy's rotation by measuring its instantaneous back and forth motions doesn't allow one to actually see things change over time," said van der Marel, the lead author on a paper in the Feb. 1 issue of the Astrophysical Journal describing and interpreting the results.
"By using Hubble to study the stars' motions over several years, we can actually for the first time see a galaxy rotate in the plane of the sky."
More Information: Third-epoch Magellanic Cloud Proper Motions. II. The Large Magellanic Cloud Rotation Field in Three Dimensions: Roeland P. van der Marel1 and Nitya Kallivayalil - iopscience.iop.org/0004-637X/781/2/121/article
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