Newswise — In March, scientists announced that a flurry of eight objects that look to be newfound dwarf galaxy satellites of the Milky Way have turned up in observations by the Dark Energy Survey, an international astronomy project that aims to explain the universe's accelerating expansion. These miniature galaxies – the first discovered in a decade – shine with a mere billionth of our galaxy's brightness and each contain a million times less mass. Belying their diminutive stature, dwarf galaxies are big news for scientists.
Why? As the Director of the Dark Energy Survey, Josh Frieman, and astrophysicists Alex Drlica-Wagner and Andrea Albert, told The Kavli Foundation on May 18, 2015, the continued study of dwarf galaxies could reveal the identity of the particles composing "dark matter." Furthermore, dwarf galaxies could help explain the development of the universe's grand galactic architecture over cosmic history.
"These new dwarf galaxies are perhaps some of the earliest structures that formed in the universe and are the building blocks of the large-scale structure we see today,” said Frieman, Professor of Astronomy and Astrophysics at the University of Chicago and a member of the Kavli Institute for Cosmological Physics (KICP). "So these dwarf galaxies are potentially an indicator of what happened in the early phases of structure formation."
Because the vast majority of material in dwarf galaxies should be dark matter—a mysterious substance accounting for nearly 80 percent of all matter—these mini-galaxies have emerged as prime targets for gathering potential clues about its composition. Some theories suggest dark matter particles and antiparticles should produce telltale gamma rays when they collide with each other.
Accordingly, scientists recently used the Fermi Gamma-Ray Space Telescope to study the dwarf galaxy candidates found by the Dark Energy Survey, as well as a group of dwarf galaxies already on the books. The telescope detected no significant gamma-ray signals from either set of dwarf galaxies, however, leaving scientists still in the hunt for dark matter's constituents.
"We have not seen dark matter yet. However, we are just starting to scratch the surface," said Albert, Research Associate at the SLAC National Accelerator Laboratory and the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University. "There are models for more massive dark matter particles than we can now detect with Fermi, and which are just as viable. So I think that if dark matter is hiding, we are narrowing in on it, and that's exciting."
Besides dwarf galaxies, the center of the Milky Way galaxy stands as a promising source of gamma rays that may signal dark matter annihilation. But the presence there of a densely packed stars, dust clouds and energetic stellar remnants—not to mention a supermassive black hole—complicate the analysis of these signals. Dwarf galaxies could therefore serve as key independent references for intriguing Milky Way gamma-ray signatures.
"The center of our Galaxy is a very active region, and there are loads of gamma rays coming from there. The issue is that when you see these gamma rays, it could just be that you don't fully understand how our Galaxy's center is producing them astrophysically," said Drlica-Wagner, the Schramm Experimental Fellow at the Fermilab Center for Particle Astrophysics and a KICP Associate fellow. "Dwarf galaxies are extremely important because you can use them as a cross-check on any interesting signals you see from the Galactic center."
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Read the full conversation with Frieman, Drlica-Wagner and Albert on The Kavli Foundation website: http://www.kavlifoundation.org/science-spotlights/dwarf-galaxies-loom-large-quest-dark-matter
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