Spotting the faintest objects in the sky is a job for one of the most powerful digital cameras. Fortunately, scientists working on the Dark Energy Survey (DES) have access to one. As if showing off its superior star-spotting abilities, it has found eight new incredibly faint objects in the sky. These objects, despite their unremarkable appearance, could be valuable tools for scientists to study dark matter.
The objects are known as satellite dwarf galaxies, and they are mere specks compared to fully formed galaxies. The Milky Way, for example, hosts an estimated 100 billion stars, whereas a dwarf galaxy harbors around 1,000. They are satellites because they orbit around larger galaxies, such as the Milky Way. These satellite galaxies can be extremely faint, and the DES can see the faintest of them only when they are nearby to our own galaxy. The results have been pre-published on arXiv.
Even though these dwarf galaxies might not be blinged out with lots of stars, scientists believe that they are brimming with something we can't see: dark matter. Since it is thought that dwarf galaxies are the building blocks of galaxies, it is of interest to scientists to study them since they could expose some of the secrets of galaxy formation and dark matter.
The objective of the DES is ultimately to discover the nature of dark energy: a mysterious force that causes the universe to expand. It's thought to make up 70% of the universe. During its dark energy surveys, the DES mainly directs its attention towards distant galaxies, but occasionally it spots something a little closer to home. In this case, it sighted eight faint dwarf galaxies.
Keith Bechtol from the University of Wisconsin-Madison summarized why it has taken so long to see these nearby dwarf galaxies: "DES is finding galaxies so faint that they would have been very difficult to recognize in previous surveys. The discovery of so many new galaxy candidates in one-eighth of the sky could mean there are more to find around the Milky Way."
The closest of these recently discovered dwarf galaxies is 80,000 light-years away, and the furthest is 700,000 light-years away. For comparison, the Andromeda galaxy – the nearest galaxy to Earth – is roughly 2.5 million light-years from the Milky Way.
Most of these objects have been spotted near the Large Magellanic Cloud and the Small Magellanic Cloud, two of the Milky Way's satellite galaxies. It is possible that the dwarf galaxies are satellites of the Magellanic Clouds, which themselves are satellites of the Milky Way. "That result would be fascinating," said Risa Wechsler of DOE’s SLAC National Accelerator Laboratory. "Satellites of satellites are predicted by our models of dark matter. Either we are seeing these types of systems for the first time, or there is something we don’t understand about how these satellite galaxies are distributed in the sky."
The discovery of these dwarf satellites, thought to be full of dark matter, is promising for the future of dark matter research. The next season of surveys has begun, and scientists are eager to see what discoveries pop out of the data.
"This exciting discovery is the product of a strong collaborative effort from the entire DES team," said Basilio Santiago, a member of the DES-Brazil Consortium. "We've only just begun our probe of the cosmos, and we're looking forward to more exciting discoveries in the coming years."