Plotting the Three-Dimensional Dance of Galaxies
Plotting The Three-Dimensional Dance Of Galaxies
The European Space Agency's Gaia telescope is designed to map the position and speed of a billion stars in the Milky Way and its neighboring galaxies. In fact, some of those galaxies are satellites, which whirl around our home galaxy in a complicated dance. Part of Gaia's mission is to help us understand that dance.


   
    Map of the positions and velocities of many of the Milky Way's satellite galaxies and globular... [+] clusters.
   ESA/Gaia
  


Many of these satellite galaxies actually orbit inside the halo of the mysterious, invisible dark matter that makes up most of the Milky Way's mass. For that reason, the dance of the satellites tells us about the structure of the Milky Way, along with the shared history and evolution of all the galaxies involved. The Gaia space telescope's second data release from last week allowed astronomers to map out the positions and motion of stars inside eleven satellite galaxies, along with other star clusters. The result: new estimate on the mass of the Milky Way, and a fully three-dimensional map of nearly 90 objects in orbit around our galaxy. (To see a video of these orbits, go to the release website.)
Only two of the Milky Way's satellite galaxies — the Large and Small Magellanic Clouds — are visible without a telescope, and many others are so faint we didn't know they existed before the Hubble Space Telescope was built. As a result, astronomers aren't sure if some of these dwarf galaxies (as they're called) are bound to the Milky Way by gravity, a mystery Gaia is well-suited to solve. Researchers measured the position and motion of stars in the Magellanic Clouds, along with nine dwarf galaxies and a huge number of globular clusters — bright collections of hundreds of thousands of stars orbiting the Milky Way.

According to the standard theory of galaxy formation, small galaxies formed first, then merged to make bigger and bigger galaxies, including the Milky Way. Some of the smaller galaxies stuck around, though, and those are the dwarf galaxies we see orbiting many of the bigger ones. Understanding the structure and motion of these galaxies, in other words, helps us trace the history of the Milky Way.

Gravity from the Milky Way holds these satellites in orbit, but it's a complicated situation. The Milky Way's mass is spread out over huge region of space. Most of that is the dark matter halo, which is effectively empty space: stuff can pass through the halo without colliding with anything. (Even the bright part of the Milky Way, like the part we inhabit, is still mostly empty space.) As a result, the globular clusters and most of the satellite galaxies are technically inside the dark matter halo, and determining the force of gravity holding them in orbit becomes complicated.
The way astronomers measure mass is by its gravitational effect on other objects. The new Gaia results allowed astronomers to place a lower limit on the Milky Way's mass thanks to the motion of the satellite galaxies: our galaxy is at least 980 billion times the mass of the Sun. (We don't see enough stars and other visible matter, like gas, to make up that much mass, so we know the remainder must be dark matter.) In addition, the Gaia results show how the Milky Way is pulling some of the satellite galaxies apart, and to a lesser extent how those galaxies tug back on the Milky Way. The shape of the visible part of the Milky Way, particularly the dense inner region, makes a difference to the motion of the satellites.

One interesting final note: the Gaia researchers found no evidence that the satellites revolve around the Milky Way in the same direction, or lie in the same plane the way planets do in the Solar System. That's in contrast to a few other galaxies. What that means is unclear, but that's what science is about: looking at weird phenomena and figuring out what causes them.Follow me on Twitter or LinkedIn. Check out my website. 