The gravitational waltz of the Milky Way’s satellites

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I’ve started contributing to the Forbes Science page again! This is my first new contribution, relating to the second data release from the Gaia survey telescope. (And if I can be shameless: Forbes pays according to traffic, so the more of you who share and visit and read my stuff, the better they pay me. Ahem.)

Plotting The Three-Dimensional Dance Of Galaxies

A map of the Milky Way’s satellite galaxies, globular clusters, and other objects in orbit. [Credit: ESA/Gaia]

For Forbes:

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.

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.

[Read the rest at Forbes]


Our local group of galaxies—known imaginatively as the Local Group—has two huge galaxies: the Milky Way and M31, also known as the Andromeda Galaxy. Both of these galaxies are large enough to have a number of satellites, including the substantial Magellanic Clouds and M33 (Triangulum Galaxy). However, most satellites are dwarf galaxies, very faint and relatively low mass. As a result, a moderately complete census of satellites has proven difficult even for the Milky Way, but what recent observations have found is surprising. In both cases, a number of the satellite galaxies orbit in a single plane, and at least in the case of Andromeda, they orbit in the same direction.

The Pan-Andromeda Archaeological Survey (featuring the diverting acronym PAndAS) was established to provide a high-resolution, large-scale panorama of M31 and its environs. 27 dwarf galaxies that can be unambiguously associated with Andromeda lie within the PAndAS survey region. The astronomers measured the distances and velocities of each of these galaxies, yielding a three-dimensional and dynamical view of the M31 system.

They found 15 of those satellites were arranged along a relatively thin arc from the perspective of Earth, meaning they lie close to a single plane. Further analysis revealed 13 of the 15 galaxies were also moving in a coherent pattern: those “north” of Andromeda were moving away from us, while those “south” were traveling toward us. That indicates a clear rotational pattern; the authors estimated only a 1.4 percent probability of motion like this being random chance. [Read more…]

Why do half of Andromeda’s satellite galaxies orbit in a plane?