In a first ever scientists have measured the rotation of a galaxy by measuring the movement of the stars within it. In the central part of the our neighboring galaxy, an object called the Large Magellanic Cloud (LMC), completes a rotation every 250 million years. Over a period of seven year Hubble measured the average slight motion of hundreds of individual stars in the cloud located 170,000 light-years away. Coincidentally, it takes our Sun the same amount of time to complete a rotation around the center of our Milky Way galaxy.
Disk-shaped 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.
“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.” said Roeland van der Marel, the lead author on a paper in the Feb. 1 issue of the Astrophysical Journal describing and interpreting the results.
Nitya Kallivayalil of the University of Virginia in Charlottesville, Va, who led the data analysis, added: “Studying this nearby galaxy by tracking the stars’ movements gives us a better understanding of the internal structure of disk galaxies. Knowing a galaxy’s rotation rate offers insight into how a galaxy formed, and it can be used to calculate its mass.”
Hubble is the only telescope that can make this kind of observation because of its sharp resolution, its image stability, and its 24 years in space. “If we imagine a human on the Moon,” van der Marel explained, “Hubble’s precision would allow us to determine the speed at which the person’s hair grows.”
“This precision is crucial, because the apparent stellar motions are so small because of the galaxy’s distance,” he said. “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.”
The research team used Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys to observe stars in 22 fields spread across the vast disk of the LMC, which appears in the southern night sky as an object about 20 times the angular diameter of the full moon. Arrows on the accompanying image show the predicted motion over the next 7 million years, based on the Hubble measurements.
Each field was chosen to contain not only dozens of LMC stars, but also a background quasar, a brilliant beacon of light powered by a black hole in the core of a distant active galaxy. The astronomers needed the quasars as fixed background reference points to measure the extremely subtle motion of the LMC stars.
This measurement is the culmination of ongoing work with Hubble by van der Marel and another team to refine the LMC’s rotation rate. Van der Marel began analyzing the galaxy’s rotation in 2002 by creating detailed predictions, now confirmed by Hubble, of what the rotation should look like.
“The LMC is a very important galaxy because it is very near to our Milky Way,” he said. “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.”
“Because the LMC is so nearby, it is a benchmark for studies of stellar evolution and populations. For this, it’s important to understand the galaxy’s structure,” Kallivayalil said. “Our technique for measuring the galaxy’s rotation rate using fully three-dimensional motions is a new way to shed light on that structure. It opens a new window to our understanding of how stars in galaxies move.”
In addition to the LMC’s own rotation, it is also moving around the Milky Way as a whole. In earlier science papers, the team and its collaborators used Hubble data to show that the LMC moves faster around the Milky Way than previously believed. This research has revised our understanding of how many times these neighboring galaxies might have met and interacted in the past.
The team next plans to use Hubble to measure the stellar motions in the LMC’s diminutive cousin, the Small Magellanic Cloud, using the same technique. The galaxies are interacting, and that study should also yield improved insight into how the galaxies are moving around each other and around the Milky Way.