Dark Energy Survey Reveals That The Milky Way Has Devoured 11 Other Galaxies

An infrared image of hundreds of thousands of stars crushed together in the swirling core of the Milky Way galaxy. Cooler stars are blue, redder parts are heated dust, and Sagittarius A* - the astronomical radio source at the galaxy's heart - is the white central spot. JPL/NASA

Robin Andrews 17 Jan 2018, 10:17

As part of an effort to find out more about the elusive nature of the cosmos, the Dark Energy Survey (DES) – a joint venture between the University of Chicago and dozens of other institutions across the world – was launched back in the summer of 2013.

Scanning the southern sky from the Chilean Andes ever since, the initiative has now released three years’ worth of data, and one paper in particular stands out: It reveals that the Milky Way has eaten 11 other galaxies, and its remnants can be spotted today zipping through the shadows.

Stellar streams are elongated stretches of stellar stuff. They could be remnants of globular clusters – collections of stars orbiting galactic cores – or dwarf galaxies torn apart by tidal forces exerted on them in a destructive ballet with our very own Milky Way.

As of December 2017, about 20 stellar streams were identified within the Milky Way, many of which were found by the DES’s precursor, the Sloan Digital Sky Survey. Several were also spotted in the nearby Andromeda galaxy, with 10 more spotted elsewhere in the Local Group: the 54 or so nearby galaxies, all orbiting around a point located somewhere between Andromeda and the Milky Way.

The DES – using its 570-megapixel Dark Energy Camera – has identified 11 new stellar streams, and their positioning and trajectories suggest that they were once galaxies a little smaller than our own that, over astronomical timescale, were torn asunder.

The new streams stand out in more ways than one. "The stellar streams we discovered are generally farther away and fainter than previously known streams," lead author Nora Shipp, a graduate student at the University of Chicago, told IFLScience.

The discovery’s paper – put together by researchers from 45 different scientific institutions – explains that these stellar streams could be used to help explain other galactic oddities. As pointed out by Shipp, this includes the Milky Way’s enigmatic stellar halo, a disparate collection of (mostly) very old stars orbiting the galaxy at some distance.

The Dark Energy Camera is mounted at the Cerro Tololo Inter-American Observatory (CTIO) in the Chilean Andes. DES

Rather beautifully, as noted by an official press release, there is no naming convention for stellar streams, so schoolchildren in Chile and Australia will get to name them. They’ll be picking from aquatic terminology in native languages from northern Chile and aboriginal Australia.

Presenting their work at a gathering of the American Astronomical Society in Washington DC, they re-asserted the yet-grander purpose of their work. After all, it’s not called the Dark Energy Survey for nothing.

When we think of matter, we think of baryonic matter – the stuff we can interact with and detect directly. This only makes up 5 percent of the universe, though, with the rest being made of a litany of mysteries.

About 25 percent is comprised of dark matter, which does not emit energy we are able to detect, but which is suspected of exerting a gravitational force on objects. Its existence, yet to be directly confirmed, is needed to explain why stars move as they do, and why galaxies spin as they do.

Galactic spiral arms, at the speed they rotate, should shed a heck of a lot of matter off into space, but they don’t. With the gravitational effects of normal – at least to us – matter taken into account, we still require more gravity to explain their behavior, and that is where dark matter is suspected to play a role.

The newly discovered, provisionally named stellar streams, along with four others - ATLAS, Molonglo, Phoenix and Tucana II - already known about.

Our universe’s expansion is continuously accelerating, which is odd: the gravitational attraction between galaxies and so on should be strong enough to slow it down over time. Its continual expansion is thought to be down to another repelling force we cannot yet detect, and that’s what’s referred to as dark energy, which makes up the final 70 percent.

DES has been tracking over 400 million astronomical objects to better understand the cosmic expansion, as well as the forces that bind it together. The identification of these devoured galactic cadavers, then, will only help advance that cause.

These streams may be the leftovers of acts of destruction, but rather remarkably, they're aiding in acts of creation too.

"Stellar streams also bring stars into the Milky Way," Shipp says, adding that "the disruption of smaller galaxies is an important component of the formation of the Milky Way stellar halo - the stars that lie outside the spiral disk of our galaxy."

A cosmic cycle of death and rebirth, captured on camera.

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