The Many Mysteries of Dwarf Galaxies
In 1519, the ships of explorer Ferdinand Magellan set off from Spain. Serving as an assistant to Magellan himself was a Venetian named Antonio Pigafetta, an astronomer, geographer, and cartographer who reported on two large patches of stars visible in the night sky during the voyage. These dim clusters—known for thousands of years to indigenous people of the Southern Hemisphere, and for at least 500 years to Arabic scientists—had only recently come to the attention of a few scholars in Europe. Pigafetta’s account brought fame to the structures, which eventually became known as the Large and Small Magellanic Clouds.
Researchers now know the duo to be dwarf galaxies that orbit around the Milky Way in much the same way that planets go around the sun. Dwarf galaxies, as their name suggests, are smaller than something like the giant spiral galaxy we inhabit—often containing less than a billion stars, as opposed to roughly 300 billion like the Milky Way. But far from being cosmic lightweights, dwarf galaxies are fascinating formations in their own right.
Known as UGC 4459, this dwarf galaxy is located approximately 11 million light-years away in the constellation of Ursa Major (The Great Bear), a constellation that is also home to the Pinwheel Galaxy (M101), the Owl Nebula (M97), Messier 81, Messier 82 and several other galaxies all part of the M81 group. Credit: ESA/Hubble & NASA Acknowledgement: Judy Schmidt (Geckzilla)
Supercomputer simulations can explain the evolution of the universe with remarkable accuracy; showing how randomly-distributed matter turned into a cosmic web of galactic superclusters arranged in long filaments separated by colossal voids. But “the reason we study dwarf galaxies is that’s where we get the most things wrong,” said astronomy graduate student Coral Wheeler of the University of California, Irvine, whose doctoral research focuses on dwarf galaxies and cosmic simulations.
These highly accurate simulations have trouble at small scales, predicting an order of magnitude more dwarf galaxies than have been found. While some researchers think better observation techniques will solve this problem, dwarf galaxies continue to confound them in other ways. This is important because dwarf galaxies are often dominated by dark matter, sometimes containing only 0.002 percent visible, ordinary matter (that stuff making up people and stars). Any dark matter theory that wants to be taken seriously will have to explain how dwarf galaxies form. Perhaps most interestingly, dwarf galaxies might be a key to complicating dark matter, pointing to the ways in which that mysterious substance interacts with itself and its surroundings.
The story of dwarf galaxies starts at the beginning of the universe. After the Big Bang, as the cosmos cooled down from a sea of ultra-hot particles and energy, matter was unevenly distributed throughout space. Denser spots had slightly more gravitational pull and therefore attracted material to them. Some spots eventually grew massive enough to form giant spherical halos. Over the course of cosmic history, these blobs spun around one another and crashed and merged to create the large-scale structure we see all around us today—galaxies collected together into clusters of galaxies which are in turn part of galactic superclusters.
Or at least that’s what we think. When astronomers simulate the processes described above, they end up with something that looks pretty much like the distribution of galaxies in the universe, except for the dwarf galaxies. Computer models say that we should see hundreds of them around the Milky Way but, so far, we’ve only found about 25. An important thing to note is that, until recently, most supercomputer simulations focused solely on dark matter. Considering that dark matter comprises approximately 85 percent of the matter in the universe, this can be an effective simplification when processing power is limited. Visible stars and galaxies, gravitationally attracted to the dark matter, are mostly being tugged around by these more powerful forces.
Still, as early as 1999 some researchers started asking “Where are all the missing galactic satellites?” The discrepancy grew worse as supercomputers got better, said Wheeler. As the simulations’ resolution improved, more and more virtual dwarf galaxies kept popping up. Simulations have since become more realistic, incorporating the effects of violent processes such as supernova explosions, which can blast out energy and stir up surrounding matter. Yet the difference between theory and observation has remained.
Many astronomers think the dwarf galaxies aren’t really missing—they’re just invisible. Considering how small and faint they are, we can only see dwarf galaxies in our local region of the universe. Even those are tough to detect, often blocked by the bright glow of the Milky Way. Telescopes have only surveyed a small part of the sky at the level of detail needed to spot dwarf galaxies. In fact, when astronomers expanded their searches during the last few years, they more than doubled the number of known dwarf galaxies, finding some that are among the faintest, most dark-matter-dominated galaxies in the universe. Researchers suspect that there could be “100 missing satellite galaxies in the Local Group, lurking just beyond our ability to detect them, or simply inhabiting a region of the sky that has yet to have been surveyed,” according to a 2010 paper.
So are the dwarf galaxies then accounted for? Perhaps, though “until we find them, or have more solid evidence that they’re there, it’s still an issue,” said Wheeler.
In the meantime, a related problem also pitting simulation against reality has sprung up. Dwarf galaxies are presumed to occupy areas where dark matter hangs out. The visible stars essentially map out the dark matter’s location. Furthermore, astronomers can tell the amount of dark matter in a galaxy by the speed at which its stars rotate—the more dark matter in the galaxy’s core, the more mass there is to tug on the stars and make them orbit faster. But a conundrum called the “Too Big to Fail” problem appeared when researchers tried to match the dark matter structures in their simulations with actual dwarf galaxies. The real-world dwarf galaxies were sitting inside of much smaller dark matter blobs than theoretically predicted.
This means there could be massive chunks of dark matter that, for whatever reason, contain little to no visible stars. Researchers have suggested different ways to explain this finding. Perhaps there is something unaccounted for in computer simulations. Some of the most recent ones (after incorporating star formation dynamics and other physical processes) seem not to suffer from either the missing dwarf galaxies or Too Big to Fail problems, though Wheeler cautions against taking any single simulation as the final word on these matters.
This dramatic image shows the NASA/ESA Hubble Space Telescope’s view of a dwarf galaxy known as NGC 1140, which lies 60 million light-years away in the constellation of Eridanus. This small galaxy is undergoing what is known as a starburst. Credit: ESA/Hubble & NASA
Another intriguing possibility is that dark matter itself is more complicated than we currently think. Visible matter—atoms and molecules and such—is subject to all kinds of forces, for instance exchanging photons as part of an electromagnetic interaction. Some theorists suspect that dark matter could have analogous interactions, with particles of dark matter exchanging “dark photons” or other “dark forces.” Dwarf galaxies thus make ideal dark matter laboratories. Already, astronomers have watched them to see if dark matter particles might be annihilating in their centers, producing telltale gamma radiation, though these searches have yet to turn up anything. It seems that dwarf galaxies will continue to hold onto their secrets for a little while longer.