The Milky Way galaxy now officially has new neighbours. Nine dwarf satellites, including three tiny galaxies, in orbit around our home galaxy were spotted by astronomers at the University of Cambridge in the skies of the southern hemisphere. The galaxies were found near the Large and Small Magellanic Cloud -- the two largest and most well-known dwarf galaxies in orbit around the Milky Way.
"The discovery of so many satellites in such a small area of the sky was completely unexpected," said lead author Dr Sergey Koposov of Cambridge's Institute of Astronomy. "I could not believe my eyes."
Discovered in newly released imaging data from the Dark Energy Survey, the find consists of three confirmed dwarf galaxies and six objects that could be either be dwarf galaxies or globular clusters -- the difference being that the stars in globular clusters are not held together with dark matter.
Dwarf galaxies are the smallest of all the observed galactic structures, sometimes as small as just 5,000 stars -- compared to the Milky Way's estimated 200 to 400 billion stars. It is also estimated that they contain up to 99 percent dark matter, and just one percent observable matter, which makes them perfect for testing dark matter models.
"Dwarf satellites are the final frontier for testing our theories of dark matter," said study co-author Dr Vasily Belokurov of the Institute of Astronomy. "We need to find them to determine whether our cosmological picture makes sense. Finding such a large group of satellites near the Magellanic Clouds was surprising, though, as earlier surveys of the southern sky found very little, so we were not expecting to stumble on such treasure."
The closest of the three dwarf galaxies, 97,000 light-years away and located in the constellation Reticulum, is in the process of being pulled apart by the Milky Way's enormous tidal forces. The farthest and brightest, 1.2 million light-years away in the constellation Eridanus, is right on the edge of the Milky Way and is on the verge of being pulled in.
"These results are very puzzling," said co-author Wyn Evans of the Institute of Astronomy. "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."
Current estimates put the possible number of dwarf galaxies in orbit around the Milky Way in the hundreds, but they are very hard to find, since they are so faint. So far, fewer than 30 dwarf satellite galaxies have been identified.
The full study was published in The Astrophysical Journal. "Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds" can be found online.