First-Ever Precise Calculation of the Large Magellanic Cloud's Rotation Rate

February 19, 2014

A recent study makes the first precise calculation of the rotation rate of a galaxy. By determining the rotation speed of the Large Magellanic Cloud galaxy, astronomers can hope to better understand the evolution of its internal structure and evolution of galaxies in general.

Rotation of the Large Magellanic Cloud galaxy calculated by observing the movement of its stars (red arrows).

Rotation of the Large Magellanic Cloud galaxy calculated by observing the movement of its stars (red arrows). Credit: NASA, ESA, A. Feild and Z. Levay (STScI), Y. Beletsky (Las Campanas Observatory), and R. van der Marel (STScI).

You might have thought that having observed galaxies as far as 10 billion light-years away, we would have already known something as the exact rotation speed of the Large Magellanic Cloud (LMC) galaxy which is "only" 165,000 light-years away.

Well, it took seven years of observing hundreds of the LMC's stars to find out that the central part of the galaxy takes 250 million years to rotate once. This is first such discovery, published in the Astrophysical Journal.

The Large Magellanic Cloud is our own galaxy's satellite. It is roughly 1/100 as massive as the Milky Way and has a diameter of 15,000 light-years. Although the LMC is considered to be irregular type galaxy, it has a bar in its centre, suggesting that it used to be a spiral galaxy before it was torn apart by gravitational pull from our galaxy and Small Magellanic Cloud galaxy.

We are still learning about evolution of galaxies by observing very distant (and thus old) galaxies when the Universe was still young and comparing them to different types of galaxies today. Observing the LMC is crucial because it is near enough to provide enough detail, but far enough and small enough to observe in its entirety.

"The LMC is a very important galaxy because it is very near to our Milky Way," said Roeland van der Marel, of the Space Telescope Science Institute (STScI), Baltimore, USA. He is the study's lead author. "Studying the Milky Way is very hard because everything you see is spread all over the sky. It's all at different distances, and you're sitting in the middle of it. Studying structure and rotation is much easier if you view a nearby galaxy from the outside."

Even though it is in our neighbourhood, the movement of the LMC's stars is only possible to measure by Hubble Space Telescope and it took 7 years for stars to move enough to obtain decisive data.

"This precision is crucial, because the apparent stellar motions are so small because of the galaxy's distance," said van der Marel. "You can think of the LMC as a clock in the sky, on which the hands take 250 million years to make one revolution. We know the clock's hands move, but even with Hubble we need to stare at them for several years to see any movement."

Galaxies come in three types – spiral, elliptical and irregular. This discovery is a stepping stone to our understanding how galaxy's rotation speed, mass, age and position relative to other galaxies affect their shape.



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