Stars Map Dark Matter in Dwarf Galaxy

Cosmos by John Hussey

A combo of Hubble and Gaia data reveal the distribution of dark matter in a tiny galaxy by tracking the galaxy’s stars.

The Sculptor dwarf galaxy has very few stars, making it immensely difficult to detect (it’s the faint concentration of stars at the center of this image).

ESA / Hubble / Digitized Sky Survey 2

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Scientists have tracked the motions of stars in the Sculptor dwarf galaxy, which orbits the Milky Way roughly 300,000 light-years from Earth. The results, published in the December issue of Nature Astronomy, are exciting astronomers not for the stars themselves but for what the stellar motions trace: dark matter.

Davide Massari (University of Groningen and Leiden University, The Netherlands) and colleagues combined observations taken by the Hubble Space Telescope more than a decade ago with the first data release from the Gaia satellite, showing how 126 stars move along the plane of the sky (sideways, from our perspective). Combining these observations with previous measurements of the stellar motions along our line of sight, they obtain three-dimensional velocities. Finally, picking out 15 objects for which they have the best measurements, the team mapped out the unseen dark matter halo that guides the stars’ orbital trajectories.

The popular theory of “cold” (aka, slow-moving) dark matter says that the mysterious particles ought to gather at the center of a galaxy like the peak of a dark whipped cream. Nowhere should this effect be more visible than in dwarf galaxies — they’re dark matter-dominated and largely lack the sources that could confuse measurements, such as pulsars and supernovae. But results to date have had astronomers mired in what Massari and colleagues call “a longstanding unresolved debate.”

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The new measurements show that the Sculptor dwarf galaxy might actually have the predicted peak of dark matter. But it hasn’t settled the debate: the sample of stars is small and concentrated at one location in the galaxy, and the authors acknowledge that the observations don’t rule out the non-peak scenario. Nevertheless, the observations show the power of Gaia observations — the next data release, due in April 2018, will include on-the-sky motions for a much larger sample of stars.


By: Monica Young


Cosmos by John Hussey


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