Stars' Clockwork Motion Captured in Nearby Galaxy
Image Credit: NASA, ESA, A. Feild and Z. Levay (STScI), Y. Beletsky (Las Campanas Observatory), and R. van der Marel (STScI)
Science Credit: NASA, ESA, R. van der Marel (STScI), and N. Kallivayalil (University of Virginia). Release Images
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). Acknowlegment: S. Guisard (http://sguisard.astrosurf.com/). Release Videos
Using
the sharp-eyed NASA 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.
The Hubble team, composed of Roeland van der Marel of the Space
Telescope Science Institute in Baltimore, Md., and Nitya Kallivayalil
of the University of Virginia in Charlottesville, Va., used Hubble to
measure the average motion of hundreds of individual stars in the LMC,
located 170,000 light-years away. Hubble recorded the stars' slight
movements over a seven-year period.
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.
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.
"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."
Kallivayalil, 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.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Roeland van der Marel
Space Telescope Science Institute, Baltimore, Md.
410-338-4931
marel@stsci.edu
Source: Hubble Site
