Nine Dwarf Galaxies Discovered Orbiting Around the Milky Way

This illustration maps out the previously discovered dwarf satellite galaxies (in blue) and eight of the newly discovered candidates (in red) as they sit outside the Milky Way / Yao-Yuan Mao, Ralf Kaehler, Risa Wechsler (KIPAC/SLAC).
Janet Fang 11 Mar 2015, 23:39

Astronomers analyzing imaging data from the Dark Energy Survey have discovered nine new potential dwarf galaxies orbiting our own galaxy—the largest number ever discovered all at once. The findings, published as two studies available on arXiv, may help explain some of the mysteries about dark matter, that invisible substance holding galaxies together. 

Dark matter makes up 25 percent of all matter and energy in the universe, and its presence is only known through its gravitational pull. Dwarf galaxies contain up to 99 percent dark matter, leaving just one percent of observable matter. Three of the new satellites are definitely dwarf galaxies, and the others could be dwarf galaxies, but they might also be globular clusters. These are like dwarf galaxies except they aren’t held together with dark matter. 

These are the first dwarf galaxies to be discovered since dozens were spotted back in 2005 and 2006 above the northern hemisphere. The nine new satellites were found in a small patch of sky above the southern hemisphere near the Large and Small Magellanic Clouds, the largest dwarf galaxies in the Milky Way’s orbit. These new objects are about a million times less massive and a billion times dimmer than the Milky Way, which contains hundreds of billions of stars. That’s why they’ve been so hard to find. 

Six of the 9 newly discovered satellites are labeled in this image with the Magellanic Clouds and the Paranal Observatory in the Atacama Desert in Chile. The other three are just outside the field of view.

Their distances range from about 95,000 light-years to 1.2 million light-years from Earth. The nearest one is located in the constellation of Reticulum (or the Reticle) halfway to the Magellanic Clouds, and it’s in the process of being torn apart by tidal forces. The most distant of the objects is in the constellation of Eridanus (or the River) right on the fringes of the Milky Way, and it’s about to get pulled in. 

“These results are very puzzling,” Cambridge’s Wyn Evans says in a news release. “Perhaps they were once satellites that orbited the Magellanic Clouds and have been thrown out by the interaction of the Small and Large Magellanic Cloud. Perhaps they were once part of a gigantic group of galaxies that—along with the Magellanic Clouds—are falling into our Milky Way galaxy.”

The Dark Energy Survey, headquartered at the Fermi National Accelerator Laboratory, seeks to photograph the southern sky in unprecedented detail using its 570-megapixel Dark Energy Camera. Mounted on a telescope at the Cerro Tololo Inter-American Observatory in the Andes Mountains in Chile, the camera makes it possible to see galaxies up to 8 billion light-years away. These new discoveries were made independently by Cambridge researchers led by Sergey Koposov and a Fermilab team led by Alex Drlica-Wagner.

“Dwarf satellites are the final frontier for testing our theories of dark matter,” Vasily Belokurov of Cambridge explains. “We need to find them to determine whether our cosmological picture makes sense.”

[Via Cambridge & Fermilab]

Images: Yao-Yuan Mao, Ralf Kaehler, Risa Wechsler/KIPAC/SLAC (top), ESO/Y. Beletsky & V. Belokurov, S. Koposov/IoA, Cambridge (middle)

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