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Chaos mixes the Milky Way

Speed read
  • Stellar streams give evidence of galaxies consumed by the Milky Way.
  • Chaos theory helps explain how these streams will evolve.
  • Supercomputers unravel these streams and promise to paint our dark matter halo.

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 <strong> Cosmic subtraction.</strong> Whereas the Milky Way was previously thought to have four arms, infrared images from NASA's Spitzer Space Telescope have helped scientists discover that our galaxy's spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars. Courtesy Robert Hurt; NASA.Our Milky Way has evolved to its present shape after billions of years. Dark matter makes up its unseen bulk, but it now seems that only by embracing chaos will we know its true size. To help correctly track the Milky Way's dark matter halo, US National Science Foundation (NSF) -funded cosmologists employed supercomputers to model these irregular orbits.  

Think of our planet as a tiny blue fish splashing in a virtually boundless starry ocean. We travel with neighboring fish around a medium sized star, which in turn swims in one of the arms of a barred spiral galaxy we call the Milky Way. These spiral arms appear to be connected strands, but are really discontinuous star nurseries, whipped into line at a cruising speed of roughly 220 kilometers (136 miles) per second.

“The Milky Way is eating many satellite galaxies that leave a sort of trail of breadcrumbs as they get gobbled up,” says Adrian Price-Whelan.

With a diameter of around 100,000 to 180,000 light years, the Milky Way transports 200-400 billion stars and 100 billion planets. A large vessel indeed, this galaxy is just one of more than 100 billion galaxies in the universe. Galaxies are thought to grow through absorption — like cosmic leviathans, they consume smaller galaxies. Absorption takes billions of years, and the streams of stars left behind is the evidence marking the remainders of satellite galaxies ripped apart by the Milky Way’s cosmic hunger.

<strong> Star history. </strong>Three newly-discovered streams arcing high over the Milky Way galaxy are remnants of cannibalized galaxies and star clusters. Though only about 150 globular clusters orbit the Milky Way today, they may once have numbered in the thousands. Once crowded so closely together that they could sometimes actually collide, these stars are now separated by many light-years, trailing one another at half a million miles an hour. Courtesy NASA.

“The Milky Way is eating many satellite galaxies that leave a sort of trail of breadcrumbs as they get gobbled up,” says Adrian Price-Whelan, NSF graduate research fellow at Columbia University. “The tidal forces from the mass in the Milky Way cause the smaller galaxies to unravel and create long, thin streams of stars called tidal streams. These trace out the future and past orbit of the satellite galaxies, and from a single snapshot in time, we can learn about the orbit of the smaller galaxy and therefore study the distribution of matter around the Milky Way.”

According to our current understanding, the largest percentage of the cosmos remains unseen (dark), yet is indirectly observable by the effect it wields on visible matter. In similar fashion, we can deduce the mass of our sun from the effect seen on orbiting planets. The Lambda Cold Dark Matter (∆CDM) theory incorporates this understanding of dark matter and offers remarkably precise large-scale predictions of properties of the universe. One of the predictions is that galaxies live in halos of dark matter.

These halos are thought to be triaxial, but this prediction awaits verification when the European Space Agency Gaia satellite completes its galactic survey. Gaia’s five-year mission ends in 2018, and by measuring the velocities of hundreds of millions of stars around the Milky Way, will give cosmologists a 3D look at the exact shape of our spiral home. But for all of Gaia’s promise, unless chaos theory is integrated into orbital calculations, scientists might not be able to trace the true shape of the dark matter halo surrounding our galaxy.

<strong>Overarching via lactea. </strong> The Milky Way arch emerging from the Cerro Paranal, Chile, on the left, and sinking into the Antofagasta's night lights. The Magellanic Clouds are visible on the left side, and a plane has left a visible trace on the right, along the Vista enclosure. Courtesy Bruno Gilli; ESO. Creative Commons Attribution 4.0.If some of the stars in these tidal streams were subject to chaotic orbits, their streams would 'fan' out more quickly, making them unrecognizable to Gaia as stellar streams. However, Price-Whelan notes, thus far we’ve only observed thin tidal streams in the Milky Way — so either the dark matter halo around the Milky Way doesn’t contain chaotic orbits, or the thin streams we observe are merely the remaining streams after chaotic ones 'fanned' out long ago.

If this is the case, “there could be many more streams that formed on chaotic orbits that have now dispersed too much for us to detect them. This would also mean that the thin streams we see trace out the regular orbits around the galaxy, which would place limits on the possible configurations of dark matter around the Milky Way,” he says.

Price-Whelan recently published research in the Monthly Notices of the Royal Astronomical Society, where his team simulated chaotic stellar streams under the ∆CDM. To study how many orbits are expected to be chaotic and where these orbits are, his team had to calculate tens of thousands of orbits for hundreds of thousands of time steps.

To meet this challenge, Price-Whelan looked for a computer large enough to weather the numerical storm incurred by the tidal stream simulations. The problem lends itself to a parallel approach, so using NSF-funded Extreme Science and Engineering Discovery Environment (XSEDE) resources, researchers distributed the individual orbit integrations over many nodes. They jumped aboard Stampede, one of the XSEDE compute clusters, and Columbia University's Hotfoot and Yeti compute clusters.

Milky Way tour. The clearest infrared panorama of our galactic home ever made, courtesy of NASA's Spitzer Space Telescope. Navigate the panaroma with the Spitzer GLIMPSE360 web tool.

Price-Whelan’s results indicate that tidal streams around the Milky Way exist only on regular orbits, not chaotic ones, and can thus provide a map to the regular regions of the Milky Way. This suggests a promising new direction using tidal streams to constrain the distribution of dark matter around our galaxy. Cosmologists can now look to the Gaia survey with a new sense of confidence: Understanding how tidal streams form in the Milky Way provides a measure of the galaxy’s gravitational field over vast distances.

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