“Pulling & Twisting” –Ongoing Contortions from Milky Way’s Ancient Collision with Neighboring Dark Matter Halo


"Pulling & Twisting " --Ongoing Contortions from Milky Way's Ancient Collision with Neighboring Dark Matter Halo


The long-held belief that the Milky Way, harboring Earth and our solar system, is relatively static has been dispelled. In a violent, dynamic dance between two galaxies, the dark-matter halo of Large Magellanic Cloud (LMC) –a satellite galaxy one-hundredth as massive as the Milky Way, visible as a faint cloud in the southern hemisphere’s night skies – as observed by its namesake, the 16th century Portuguese explorer Ferdinand Magellan–is pulling and twisting the Milky Way spiral-shaped disc of stars and planets at 115,200 kilometers per hour towards the constellation Pegasus.

“This discovery definitely breaks the spell that our galaxy is in some sort of equilibrium state. Actually, the recent infall of the LMC is causing violent perturbations onto the Milky Way,” said Jorge Peñarrubia, Personal Chair of Gravitational Dynamics, University of Edinburgh School of Physics and Astronomy. “Understanding these may give us an unparalleled view on the distribution of dark matter in both galaxies.”

Scientists believe the LMC (image below) crossed the Milky Way’s boundary around 700 million years ago – recent by cosmological standards – and due to its large dark matter content it strongly upset our galaxy’s fabric and motion. The effects are still being witnessed today and should force a revision of how our galaxy evolved, according to astronomers at the University of University of Edinburgh, who found to their surprise that the Milky Way was not moving towards the LMC’s current location, as previously thought, but towards a point in its past trajectory.


LMC Galaxy


Using a sophisticated statistical model the Edinburgh team calculated the speed of the Milky Way’s most distant stars, the University of Edinburgh team discovered how the LMC warped our galaxy’s motion. They found that the LMC, powered by its massive gravitational force, is moving away from the Milky Way at the even faster speed of 370 km/s, around 1.3 million kilometers per hour.

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An earlier, 2007 study by the Harvard-Smithsonian Center for Astrophysics suggested that there are two possible explanations for these high speeds. One, that the mass extent of the Milky Way is larger than previously thought. If the LMC is gravitationally bound to the Milky Way, then the Milky Way must be much more massive than previous data suggested. The excess mass would pull on the LMC, keeping it “close at hand.” Two, that the LMC is gravitationally bound to the Milky Way. If previous calculations of the Milky Way’s mass are accurate, then the Milky Way is not massive enough to hold onto its companions. In a few billion years, it will escape from the Milky Way.

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The LMC and its companion, Small Magellanic Cloud (SMC), may not be true companions of the Milky Way, but perhaps travelers just passing through the neighborhood, suggested astronomer Nitya Kallivayalil then with the Harvard-Smithsonian Center for Astrophysics.

Astronomers now intend to find out the direction from which the LMC first fell in to the Milky Way and the exact time it happened, which will reveal the amount and distribution of dark matter in the Milky Way and the LMC with unprecedented detail.

“Our findings beg for a new generation of Milky Way models, to describe the evolution of our galaxy,” said Michael Petersen, lead author. “We were able to show that stars at incredibly large distances, up to 300,000 light-years away, retain a memory of the Milky Way structure before the LMC fell in, and form a backdrop against which we measured the stellar disc flying through space, pulled by the gravitational force of the LMC.”

The first accurate 3D map of the Milky Way shown at the top of the page reveals its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences have used 1339 ‘standard’ stars to map the real shape of our home galaxy. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)

The Daily Galaxy, Sam Cabot, via University of Edinburgh

Image credits: LMC via Harvard CfA