A close-up view of a dwarf galaxy. The diffuse collections of stars generally do not contain many gamma-ray sources.
Like moons orbiting a planet, smaller bodies orbit the Milky Way. Known as dwarf galaxies, they may be faint enough to escape detection – it’s not known how many there are in total, and new dwarfs are still being detected. One such dwarf galaxy has been discovered in recent weeks using data from the Dark Energy Survey, an experiment that scans the southern sky to learn more about the accelerated expansion of the Universe (the experiment’s name comes from the mysterious dark energy that causes acceleration).
Known as Reticulum 2, the dwarf is located about 98,000 light-years from Earth, making it one of the closest discovered satellites to the Milky Way. But that’s not the most exciting feature. The mini-galaxy appears to be emitting a strong gamma-ray signal, a research team concludes in an article submitted to the journal Physical assessment letters. That’s surprising for a dwarf, since they’re usually devoid of the objects that typically produce gamma rays. Although it is still too early to say for sure what the source of the gamma rays is, the authors have come to a very intriguing conclusion for now: destruction of dark matter.
Dwarves and dark matter
Like their larger counterparts, dwarf galaxies rest in a spherical blob or halo of dark matter that makes up most of the galaxy’s mass. In the case of the Milky Way’s satellites, their halos rest within the Milky Way’s own larger halo, making them subhalos.
Sometimes these subhalos can exist without visible galaxies in them. The reason for this phenomenon is still somewhat of a mystery, but it was originally considered a possible solution to a problem for the leading dark matter model: there were fewer visible dwarfs orbiting the Milky Way than predicted. Now that more such galaxies have been discovered, it remains a puzzle, but no longer poses a challenge to our models of dark matter.
But a new form of the problem has emerged. In the empty subhalos, mass tends to clump more toward the centers than in the subhalos containing galaxies. This Also challenges the leading dark matter model. We recently went through some of the possible solutions, and one of the most promising is the so-called self-interacting dark matter (SIDM).
It is widely believed that dark matter does not interact much with itself. That is, the individual particles are thought to collide infrequently. After all, we know they don’t interact with particles via electromagnetism, the force that holds all Earthly objects together and prevents you from sinking through the floor right now. But there is a chance that dark matter has a weak interaction, meaning it interacts via the weak nuclear force, allowing occasional collisions.
If dark matter is consistently interacting with itself, this could explain the so-called missing satellite problem described above. Energy can be redistributed among the dark matter particles as they collide, keeping the dwarf’s core density constant and the observations consistent. The question was whether evidence could be found for self-interacting dark matter.
What does all this have to do with the newly discovered dwarf, Reticulum 2, and its gamma-ray emissions?
Reticulum 2 and dark matter
The study’s authors think that Reticulum 2’s gamma rays are likely produced by colliding dark matter particles. The strong gamma-ray signal indicates that it is unlikely to be the result of other known objects that produce gamma rays, such as black holes and pulsars. “Something toward this dwarf galaxy is emitting gamma rays,” said Alex Geringer-Sameth, a postdoctoral research associate in CMU’s Department of Physics and the paper’s lead author. “There is no conventional reason why this galaxy should be emitting gamma rays, so it may be a signal for dark matter.”
And there’s no question that such a gamma-ray signal could imply dark matter. “If you see gamma rays in a dwarf galaxy, that would be a good way to show that you see dark matter,” said Neal Weiner, a New York University particle physicist who studies dark matter. New York Times.
Researchers have long regarded dwarf galaxies as the ideal places to look for such a gamma-ray signal. Larger galaxies have more dark matter, so their signal could be stronger, but they also have many more objects such as black holes that can produce gamma rays, making it more complicated to disentangle which (if any) are dark matter.
Previous studies have looked for a gamma-ray signal from dwarf galaxies such as Reticulum, but have found no significant gamma-ray emission. That didn’t rule out the possibility that gamma rays are produced by self-interacting dark matter, but it did place limits on how often dark matter particles could collide. Nevertheless, it’s fair to ask: if Reticulum’s signal is indeed the result of dark matter, why haven’t similar signals been detected from other dwarfs?
“We haven’t detected anything from other dwarfs because most of the dwarf galaxies are farther away (with the exception of one or two), and they may not be as large,” said Savvas Koushiappas, a physicist at Brown University and one of the authors of the paper. the article. Ars said. “But we have to be careful here, because at the moment we don’t have a measure of the distribution of dark matter in this new object (we made a prediction for it based on the gamma-ray flux).”
Such a measurement would normally be made by observing the motions of the galaxy’s stars. But since that work hasn’t been done yet, the researchers estimated Reticulum’s halo mass based on gamma rays alone. The more dark matter is present, the greater the chance that the particles will collide with each other. And the more collisions there are, the more gamma rays will be produced. This situation allowed the researchers to calculate the mass of dark matter.
What is Dark Matter?
While it is fairly certain that dark matter exists and makes up most of the mass of the universe, its identity is unknown. It’s probably some kind of particle, but none of the known particles in the Standard Model fit the bill. There are quite a few fascinating possibilities, including axions, sterile neutrinos, and even weird, non-particle possibilities like topological defects. But the current leading candidate is WIMPs: weakly interacting massive particles.
These theoretical particles are outside the Standard Model and got their name because they interact weakly but have a significant amount of mass. Specifically, they would produce the gamma rays seen from Reticulum. “What we showed is that the simplest explanation (cold dark matter annihilating and producing photons on its own) can be a valid hypothesis in explaining the experimental data. A WIMP can do that very well,” Koushiappas told Ars.
Does this mean that if we confirm that the gamma-ray signal comes from dark matter, it would show that dark matter is indeed WIMPs? “Maybe, maybe not,” Koushiappas said. “What needs to be done is to identify the preferred annihilation channel(s). Once we have that, it may be possible to come up with specific dark matter models that may or may not be consistent with the inferred preferred annihilation channels.”
Other possibilities
As we have seen, it is far from a foregone conclusion that the gamma-ray signal is the result of dark matter. “While those of Ret2 [gamma]-ray signal is tempting,” the authors write in their paper, “it would be premature to conclude that it has a dark matter origin.” In fact, the signals do not reach the level of certainty (five sigma) that is necessary for a discovery in particle physics; depending on how it is analyzed, the gamma-ray signal can be as low as 2.3 sigma.
This isn’t the first time a potential dark matter signal has been observed in gamma rays, but all previous signals have faded into the background with more detailed observations. So there is clearly a need for further observations before anyone gets excited.
But if it’s not dark matter, what could it be? There are a few possibilities. By far the “most mundane,” as the authors put it, is the possibility that there is another source producing gamma rays that isn’t in Reticulum at all, but just happens to be in the same direction. There is such a candidate, a quasar that is only 0.46 degrees from Reticulum 2 in the sky. The authors consider this a questionable source because that type of quasar usually does not produce such a gamma-ray signal. But they note that more work needs to be done to rule it out.
Another possibility is millisecond pulsars. These are pulsars – extremely dense neutron stars that emit radiation in beams from their poles – resembling a lighthouse – that spin so incredibly fast that they complete a full revolution in 1-10 milliseconds (hence the name). These millisecond pulsars can emit gamma rays, making them a possibility, although they are unlikely to produce Reticulum’s high-energy gamma rays.
High-energy cosmic rays can also be produced near young massive stars. If Reticulum 2 has such a population of stars, it is possible that this is the source of the gamma rays. Future spectrographic work will verify whether this is the case.
“There is a lot of work that needs to be done by the community to confirm or rule out the presence of excess gamma rays along the line of sight to Reticulum 2,” Kushiappas said. “If there is indeed an excess over what one would expect from any background, then we must find an explanation. Only in the case of dwarf galaxies is the scope of possible explanations extremely limited, as they are very silent systems (in gamma rays). Dark matter is one of those very few possibilities, perhaps the most exciting! With more data from Fermi, in addition to the upcoming updated dataset, we hope to learn more about the presence and origin of the emission.”
The arXiv. Abstract Number: 1503.02320 (About the arXiv). Will be published in Physical assessment letters.