The mystery of dark matter has dominated cosmological research for almost a century, driving astronomers’ ongoing efforts to discover the hypothetical substance that is believed to make up more than 80 percent of the total matter in the Universe. Yet, despite the many decades of theoretical and observational studies regarding its nature, this unknown form of matter has proved to be quite elusive. Nevertheless, several lines of indirect evidence accumulated through the years have indicated that whatever it is, dark matter is actually there, silently permeating the entire Universe and shaping its long-term evolution. The latest of these come in the form of several dwarf galaxies that have been recently discovered orbiting the Milky Way, which could ultimately allow researchers to narrow down the proposed explanations about the nature of dark matter.
Of all the candidates for the composition of dark matter, the leading one currently among the scientific community is a type of hypothetical particles called WIMPS, which is short for Weakly Interacting Massive Particles. According to theoretical predictions, WIMPs have no interaction whatsoever with the rest of the ordinary matter in the Universe, save for the gravitational attraction they exert on their surroundings. It is exactly because of the elusiveness of these exotic particles that dark matter has evaded a firm detection, despite the countless searches that have taken place during the last 80 years. Furthermore, one of the nagging problems that has been tied to the overall mystery of dark matter has been the apparent lack of dwarf satellite galaxies, better known as the “missing satellites problem.” More specifically, the Lambda Cold Dark Matter model, which is currently the leading cosmological model that best describes the Universe’s properties and overall evolution, postulates that there should be many hundreds of dark matter-dominated dwarf satellite galaxies orbiting the large-sized ones like the Milky Way. Yet, contrary to theoretical predictions, the Sloan Digital Sky Survey, which mapped millions of astronomical objects during the last decade in over more than one-third of the sky in great detail, has been able to detect just a couple dozen such ultra-faint dwarf galaxies around our galactic neighborhood.
This discrepancy between theory and observation has led many researchers to suggest that many aspects of the standard cosmological model, as well as our understanding of the Universe’s large-scale structure, might ultimately be flawed. On the other hand, the missing satellite problem could well turn out to be a result of observational limitations, as hinted by the recent discovery of four Cepheid variable stars at the far side of the Milky Way, which are believed to be associated with just such an ultra-faint, dark matter-dominated dwarf satellite galaxy that has gone unnoticed to date. Now, a couple of separate studies that were conducted by two independent teams of astronomers and have been accepted for publication at The Astrophysical Journal come to add more members to the Milky Way’s growing list of satellite dwarf galaxies, by reporting on the discovery of nine such candidate objects in the vicinity of our galaxy. These results come courtesy of the Dark Energy Survey, which is an ongoing sky survey in optical and near-infrared wavelengths with the aim to shed more light on the equally mysterious dark energy—the totally unknown force that permeates the Universe and causes its expansion to accelerate. To that end, the ambitious five-year collaborative international project utilises the state-of-the-art Dark Energy Camera, or DECam, which is mounted on the 4-meter Victor M. Blanco Telescope in Chile. Equipped with a total of 74 CCDs that give it a resolution of a whopping 570 megapixels, DECam has been characterised as the most powerful digital camera currently in existence. Its wide field of view of 2.2 degrees covers an area of the sky 20 times bigger than the size of the Moon as seen from Earth. It will allow astronomers to map a total of 5,000 square degrees of the southern sky and uncover previously unseen celestial objects up to 1 million times fainter than the dimmest star that can be seen with the naked eye, out to a distance of 8 billion light-years from Earth.
By analysing the data gathered during the survey’s first year of operations, two independent teams of researchers—one from the U.S. Department of Energy’s Fermi National Accelerator Laboratory in Chicago, which oversees the Dark Energy Survey project, and the second one from the University of Cambridge in the UK respectively—were able to identify nine very dim objects in a small patch of the sky at high galactic latitudes below the plane of the Milky Way, which were a billion times dimmer than the Milky Way, and a million times less massive, and contained very few amounts of stars and gas, resembling the ultra-faint, low-luminosity dwarf galaxies that had been previously detected with the Sloan Digital Sky Survey. Further photometric analysis showed that the closest of these objects, named Reticulum 2, was located at a distance of approximately 98,000 light-years, which is about half the way to the nearby Large Magellanic Cloud, while the more distant one, Eridanus 2, was more than 10 times farther away, located at a distance of 1.2 million light-years, which is approximately half the distance to the Andromeda galaxy—the nearest major galaxy to the Milky Way. “The discovery of so many satellites in such a small area of the sky was completely unexpected,” says Dr Sergey Koposov, a senior research associate at the University of Cambridge’s Institute of Astronomy and leader of the Cambridge team. “I could not believe my eyes.”
While further analysis is needed in order to confirm that all of these newly found ultra-faint objects are indeed dwarf galaxies, the researchers are confident that at least three of them are the real deal. “The nature of these systems cannot be conclusively determined with photometry alone,” writes the Dark Energy Survey team in their study. “However, judging from their low surface brightnesses, ellipticities, and/or large distances, it is likely that several are new dwarf galaxies … If spectroscopically confirmed, the Dark Energy Survey candidates may become the first ultra-faint galaxies identified outside the Sloan Digital Sky Survey footprint, and would significantly increase the population of Local Group galaxies in the southern hemisphere.”
The discovery of these new dwarf galaxy candidates could also play an important role in the search for dark matter, by allowing astronomers to gain new insights into its nature. Theoretical models predict that even though WIMP particles have no electromagnetic interaction with ordinary matter, they could nevertheless be detected via two methods: either indirectly, through their extremely rare collisions with particles of ordinary matter, or directly, by their mutual annihilation after colliding with one another. Since every WIMP particle according to theory is its own antimatter counterpart, if two of them were to meet, they would be annihilated, while producing a shower of high-energy photons in the process in the form of high-energy gamma rays, as well as streams of other secondary particles of ordinary matter.
Up till now, the search for this type of gamma-ray emissions had been focused on looking at the centers of galaxies, which are believed to harbor large amounts of dark matter. With the help of NASA’s Fermi Gamma-ray space telescope, astronomers were able to detect an excess of gamma-rays last year from the center of the Milky Way, which looked like it could have been the product of dark matter annihilation. Yet our galaxy’s center is full of more mundane astrophysical sources like pulsars and neutron stars that could mimic such a signal, making identification a real challenge. For this reason, the Milky Way’s ultra-faint satellite galaxies are ideal subjects for this search, since they are located far from the galactic center and contain almost no stars and gas.
In a recent study, which was submitted to the Physical Review Letters, a team of astronomers from the U.S. and the UK have reported just such a signal coming from Reticulum 2, the nearest of the dwarf galaxies discovered with the Dark Energy Survey. By analysing eight years’ worth of data from the Fermi space telescope, the researchers uncovered an excess of gamma rays between 2 and 10 GeV of unknown origin from the newly found dwarf galaxy, which could have been caused by the annihilation of dark matter particles. “Something in the direction of this dwarf galaxy is emitting gamma rays,” says Alex Geringer-Sameth, a postdoctoral research associate in Carnegie Mellon University’s Department of Physics and lead author of the study. “Given the way that we think we understand how gamma rays are generated in this region of the sky, it doesn’t seem that those processes can explain this signal. There’s no conventional reason this galaxy should be giving off gamma rays, so it’s potentially a signal for dark matter.”
Nevertheless, that wasn’t the conclusion of another independent study by a research team from the Fermi National Accelerator Laboratory, which looked for the same signal and found none. “While Ret2’s gamma-ray signal is tantalizing, it would be premature to conclude it has a dark matter origin,” points out Geringer-Sameth’s team. “Among alternative explanations, perhaps the most mundane is the possibility that an extragalactic source lies in the same direction. Searching the BZCAT and CRATES [star] catalogs, reveals a quasar (J033553-543026) that is 0.46 degrees from Reticulum 2. Further work must be done to determine whether this particular source contributes to the emission, though we note that at spectrum radio quasars rarely have a spectral index less than 2. One of the much-discussed astrophysical explanations for the apparent Galactic Center excess is millisecond pulsars. In the case of Reticulum 2, it is the high-energy behavior which disfavors a pulsar model, as millisecond pulsars exhibit an exponential cut-off at around 2.5 to 4 GeV. Alternatively, high-energy cosmic ray production could potentially arise in the vicinity of young massive stars. Upcoming photometric and spectroscopic analysis of Reticulum 2 will check this possibility.”
The contradicting results of these recent studies have once again shown that the search for dark matter more often than not looks like a hunt for the proverbial pot of gold at the end of the cosmic rainbow. Even so, the only way for astronomers to one day solve this overarching cosmological mystery is to continue searching, even in the face of scant evidence. “Dwarf satellites are the final frontier for testing our theories of dark matter,” says Dr Vasily Belokurov, an astrophysicist from the University of Cambridge and member of one of the teams that discovered the nine new dwarf galaxy candidates. “We need to find them to determine whether our cosmological picture makes sense. Finding such a large group of satellites near the Magellanic Clouds was surprising, though, as earlier surveys of the southern sky found very little, so we were not expecting to stumble on such treasure.”
Ultimately, it could well be that our greatest insights about the Universe might come from these faint and inconspicuous dwarf galaxies that accompany the Milky Way on its journey through the intergalactic void.