The Dark Energy Survey has now mapped one-eighth of the full sky (red shaded region) using the Dark Energy Camera on the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile (foreground). This map has led to the discovery of 17 dwarf galaxy candidates in the past six months (red dots), including eight new candidates announced today. Several of the candidates are in close proximity to the two largest dwarf galaxies orbiting the Milky Way, the Large and Small Magellanic Clouds, both of which are visible to the unaided eye. By comparison, the new stellar systems are so faint that they are difficult to “see” even in the deep DES images and can be more easily visualized using maps of the stellar density (inset). Fourteen of the dwarf galaxy candidates found in DES data are visible in this particular image. Credit: Illustration: Dark Energy Survey Collaboration
They are likely dwarf satellite galaxies, the smallest and closest known form of galaxies. Satellite galaxies are small celestial objects that orbit larger galaxies, such as our own Milky Way. Dwarf galaxies can be found with <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. Dwarf satellite galaxies, therefore, are considered key to understanding dark matter and the process by which larger galaxies form.
The main goal of the Dark Energy Survey (DES) is to better understand the nature of dark energy, key to understanding why the expansion of the universe is speeding up. DES images also contain stars in dwarf galaxies much closer to the Milky Way. The same data can therefore be used to probe both dark energy, which scientists think is driving galaxies apart, and dark matter, which is thought 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. “Just this year, more than 20 of these dwarf satellite galaxy candidates have been spotted, with 17 of those found in Dark Energy Survey data,” said Alex Drlica-Wagner of Fermi National Accelerator Lab. “We’ve nearly doubled the number of these objects we know about in just one year, which is remarkable.”….”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 newly discovered objects is about 80,000 light-years away, and the furthest roughly 700,000 light-years away. These objects are around a billion times dimmer than the Milky Way and a million times less massive. The faintest of the new dwarf galaxy candidates has about 500 stars. Most of the newly discovered objects are in the southern half of the DES survey area, in close proximity to the Large Magellanic Cloud and Small Magellanic Cloud, the 2 largest satellite galaxies associated with the Milky Way. It is possible that many of these new objects could be satellite galaxies of these larger satellite galaxies, which would be a discovery by itself.
Since 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. The 17 dwarf satellite galaxy candidates were discovered in the first 2 years of data from Dark Energy Survey, a 5 yr effort to photograph a portion of the southern sky in unprecedented detail. http://www.fnal.gov/pub/presspass/press_releases/2015/DES-Finds-Celestial-Neighbors-20150817.html
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