Eight more faint celestial objects hovering near our Milky Way galaxy have been discovered by scientists on the Dark Energy Survey, using one of the world’s most powerful digital cameras.
Evidence suggests that they, like the objects found by the same team earlier this year, are likely dwarf satellite galaxies, the smallest and closest known form of galaxies.
A satellite galaxy is a small celestial object that orbits larger galaxies, such as our own Milky Way. Dwarf galaxies can be found with fewer than 1,000 stars, in contrast to the Milky Way, an average-size galaxy containing billions of stars.
Scientists have predicted that larger galaxies are built from smaller galaxies, which are thought to be especially rich in dark matter, the substance that makes up about 25 percent of the total matter and energy in the universe. Dwarf satellite galaxies, therefore, are considered key to understanding dark matter and the process by which larger galaxies form.
Dark Energy and Dark Matter
The Dark Energy Survey (DES), as its name suggests, takes as it’s main goal the better understanding of the nature of dark energy, the mysterious stuff that makes up about 70 percent of the matter and energy in the universe. Scientists believe that dark energy is the key to understanding why the expansion of the universe is speeding up.
To carry out its dark energy mission, DES takes snapshots of hundreds of millions of distant galaxies. However, some of the DES images also contain stars in dwarf galaxies much closer to the Milky Way.
So the same data can be used to probe both dark energy, which scientists think is driving galaxies apart, and dark matter, which is believed to hold galaxies together.
Scientists can only see the faintest dwarf galaxies when they are nearby, and had previously only found a few of them. If these new discoveries are representative of the entire sky, there could be many more galaxies hiding in our cosmic neighborhood.
Alex Drlica-Wagner of the U.S. Department of Energy’s (DOE) Fermi National Accelerator Laboratory, one of the leaders of the DES analysis, said:
Dark Energy Camera
In March, Dark Energy Survey researchers and an independent team from the University of Cambridge jointly announced the discovery of nine of these objects in snapshots taken by the Dark Energy Camera, the extraordinary instrument at the heart of the DES. Two of those have been confirmed as dwarf satellite galaxies so far.
Before 2015, scientists had spotted only about two dozen such galaxies around the Milky Way. Said Keith Bechtol of the University of Wisconsin-Madison:
Because dwarf galaxies are thought to be made mostly of dark matter, with very few stars, they are excellent targets to explore the properties of dark matter. Further analysis will confirm whether these new objects are indeed dwarf satellite galaxies and whether signs of dark matter can be detected from them.
The DES Collaboration Eight Ultra-faint Galaxy Candidates Discovered in Year Two of the Dark Energy Survey FERMILAB-PUB-15-351-AE arXiv:1508.03622
Illustration Credit: Dark Energy Survey Collaboration