A team of astronomers has assembled the first fully 3-dimensional view of stellar motions in a nearby galaxy.
The Large Magellanic Cloud (LMC) — the biggest satellite galaxy of the Milky Way — appears to the naked eye as a dim gray glow nearly as large as your fist at arm’s length. Only from south of the equator can astronomers ever get a good look at it. But at a distance of just 163,000 light-years, it’s a treasure of extragalactic riches.
The LMC has always been a benchmark for studies due to its close proximity. For a century it has been a key rung on the cosmic distance ladder, providing a crucial steppingstone for determining further distances in the universe. Therefore heroic efforts have gone into determining the LMC’s distance by many different means, and the current value is considered highly accurate with only a 2% uncertainty.
Now, for the first time, astronomers have used the Hubble Space Telescope to measure the actual 3-D rotation of the LMC in space. This latest feat was achieved by combining star velocities measured in all three dimensions by different means.
It’s relatively straightforward to calculate a galaxy's rotation rate by measuring the motions of its stars. On one side of a galaxy’s spinning disk, the stars will be moving away from Earth, showing a spectral redshift (light waves are slightly stretched to redder wavelengths). On the opposite side, stars will be moving toward Earth, showing a spectral blueshift (light waves are slightly compressed to bluer wavelengths).
But this slight shift in the galaxy’s spectrum — known as the Doppler effect — only measures radial velocity, motion along our line of sight. Any motions that are relative to the plane of the sky (north-south and east-west) are undetectable in such an analysis.
These proper motions can only be seen by waiting long enough for stars to change their positions appreciably on the sky. This has long been done for nearby stars. But stars in another galaxy? That used to seem hopeless.
Now a team composed of Roeland van der Marel (Space Telescope Science Institute) and Nitya Kallivayalil (Yale Center for Astronomy and Astrophysics) has used Hubble’s phenomenal resolution, combined with the large view of its Wide Field Camera 3, to measure the average proper motions of 6,790 stars in the LMC over the course of 7 years. The stars are in 22 different fields, which were carefully chosen to contain distant quasars as fixed reference points.
The team was able to measure the proper motions of these drastically distant stars to an accuracy of 0.03 milliarcsecond per year. At best they could see a star move an apparent distance 60,000,000 times smaller than the full Moon in a year. This is the equivalent of watching an astronaut’s hair grow 5 cm over the course of a year while living on the Moon.
By combining the proper motions with existing radial velocity measurements, the team was able to provide a fully three-dimensional view of stellar motions in another galaxy for the first time. These measurements will provide insight into the LMC’s structure and formation history, as well as its gravitational interaction with the Small Magellanic Cloud (SMC), another Milky Way satellite galaxy not far away.
This interaction has warped the LMC enough that it’s hard to locate its gravitational center. Previous measurements made its visible center seem slightly offset from the center the stars orbit, which was in turn slightly offset from the center its hydrogen gas orbits. But with new three-dimensional insight, it appears the stars and gas do orbit the same gravitational center after all.
While the LMC is commonly listed as rotating in 250 million years (coincidentally similar to the Sun’s rotation period around the Milky Way), there is no single rotation rate, says van der Marel.
Stars at different distances from the center travel at much different speeds, and as stars age, they’re more likely to have been pulled around by tidal forces and interactions with each other. This increases their random motions over time and muddles their original rotation characteristics. So hot young stars tend to have more rotation (with an average circular velocity of roughly 90 km/s) than older cool stars (55 km/s).
The team also plans to use Hubble to measure stellar motions in the SMC using the same technique. This will shed further light on how the two galaxies are interacting with each other and the Milky Way.
Roeland P. van der Marel and Nitya Kallivayalil “Third-Epoch Magellanic Proper Motions. II. The Large Magellanic Cloud Rotation Field in Three Dimensions” Astrophysical Journal, 2014