Astronomers have found a set of possible dwarf galaxies near our galaxy, a discovery crucial to understanding dark matter.
Two independent teams of astronomers went panning for gold in the Southern Hemisphere sky — and they struck it rich. Between the two, the teams discovered nine possible dwarf galaxies near the Milky Way, nuggets that may pave the way for a better understanding of dark matter in our local universe.
The candidates are not yet confirmed dwarfs. Future observations will determine whether these are really dwarf galaxies or, the most likely alternative, crowded stellar cities known as globular clusters.
Keith Bechtol (University of Chicago) led a team with the Dark Energy Survey (DES), an optical and near-infrared project conducted with the 4-meter Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. Combing through the first year of images taken by the Dark Energy Camera, they came upon eight clusters of stars well outside the boundaries of the Milky Way.
A second team, led by Sergey Koposov (University of Cambridge, UK), found the same eight dwarf candidates, plus one more, in the same DES data. The ninth lies near the edge of a gap in the detector, which makes measurements of its stars’ brightnesses and colors tricky.
The dwarf candidates range in size from a mere 120 light-years to 1,300 light-years across. The nearest, Reticulum 2 at a distance of almost 100,000 light-years, is the strongest candidate. Even though it’s small (200 light-years across), it’s elongated, so it’s less likely to be a globular cluster posing as a dwarf galaxy. The second most-likely dwarf galaxy, Eridanus 2, is also stretched out and lies the farthest away at a whopping distance of 1.2 million light-years.
Both teams used different methods to search survey data for “overdensities” of stars — i.e., stellar clusters. So until future observations can be obtained, it’s still possible that some of these candidates might be globular clusters instead.
“The only true confirmation that these objects are galaxies rather than globular clusters comes from measuring their total mass,” says Bechtol’s coauthor Alex Drlica-Wagner (Fermi National Accelerator Laboratory). “Unlike globular clusters, most of the mass of galaxies resides not in their stars but in the dark matter that surrounds them."
The best way to measure the mass, Drlica-Wagner says, will be to collect spectra and measure the velocities of the stars with respect to one another. “For faint systems like those that we recently discovered, we will require some of the largest telescopes in the world to get accurate velocity measurements,” he adds.
Koposov’s team is similarly planning follow-up, including observations with the Hubble Space Telescope.
The spectra will also tell astronomers about the bulk motions of the stars — that is, their collective motion either towards or away from the Milky Way — which will reveal whether these potential dwarfs are in fact satellites of our galaxy.
Dwarf Galaxies and Dark Matter
Dwarf galaxies might hold the key to understanding the nature of dark matter. But only time will tell what, if anything, these new dwarf candidates will reveal.
Since the Fermi Gamma-Ray Space Telescope was launched in 2008, it has searched the skies for the signature of annihilating dark matter particles. This signature might be expected if dark matter consists of weakly interacting massive particles, or WIMPs. When two WIMPs meet, the standard theory goes, they produce less exotic (and more detectable) particles, as well as showers of gamma rays.
Dwarf galaxies are a great place to look for an annihilation signal because there’s not much going on — they’re less likely to host other gamma-ray sources such as black holes or pulsars. They also can have a large amount of dark matter compared with their mass in stars and gas.
So eyebrows went up when Alex Geringer-Sameth (Carnegie Mellon University) and colleagues released a paper on the arXiv on March 8th announcing a 3.7-sigma detection of gamma rays from Reticulum 2 (the nearest and strongest of the dwarf galaxy candidates). In particle-physics world, a 3.7-sigma result is considered “evidence” that’s worth following up but not “discovery,” says Fermi team member Andrea Albert (SLAC National Accelerator Laboratory).
As exciting as it would be, that evidence isn’t holding up under scrutiny. A recent publication from the Fermi collaboration uses a more sensitive (and not-yet-public) data set to investigate 15 nearby dwarf galaxies. Even though the Fermi team detected gamma-ray emissions from Reticulum 2 (at the 1.5-sigma level), there’s a 13% chance that the detection isn’t real.
“Having the significance go down like that with a more sensitive data set usually indicates a statistical fluke,” Albert says. So the gamma-ray excess seen by Geringer-Sameth’s team (which includes Koposov) is more likely a bump in a noisy background.
In fact, the Fermi team’s measurements of 15 dwarf galaxies rules out annihilation from WIMPs with masses less than 100 billion electron volts — if WIMPs do exist, they’d have to be massive indeed. (Planck satellite measurements of the cosmic microwave background don't find evidence for dark matter annihilation, but place less stringent limits than Fermi's observations.)
“In the past few years we have just started to scratch the surface,” Albert adds. WIMPs could exist at much higher masses, say 500 GeV, or perhaps even out of Fermi’s range altogether, she explains. “I'm not ready to throw in the towel.”