Universe Today
Dark matter - that mysterious, unknown stuff that's detectable only by its effect on other matter - seems to be sparking strong emissions at the heart of the Milky Way Galaxy. What scientists see could be the result of an entirely new form of dark matter.
“The energy signatures radiating from this part of our Galaxy suggest that there is a constant, roiling source of energy doing just that," said Shyam Balaji, member of a research team at King's College London, "and our data says it might come from a much lighter form of dark matter than current models consider.”
The team proposes a different form of dark matter, something lighter than WIMPs (Weakly Interacting Massive Particles). The most established theory for dark matter is that it is likely these WIMPs.
Clues at the Heart of the Galaxy
Astronomers have long known about strong emissions from the heart of the Galaxy. They emanate from a region called the Central Molecular Zone (CMZ). It surrounds the inner core of the Galaxy and teems with molecular hydrogen gas clouds. Some of the gases in the zone travel through space at hundreds of thousands of kilometers per hour. You'd expect such a place to be rich with star-forming activity since starbirth requires molecular clouds (often called HII regions). Motions of gas clouds are one part of the formula for creating stars.
The CMZ turns out to be more of a zone of avoidance when it comes to starbirth activity. Yet, it sports this energetic activity that Balaji describes. What's happening there? Balaji explained the dilemma astronomers faced trying to understand this strange region of the Milky Way. “At the center of our Galaxy sit huge clouds of positively charged hydrogen, a mystery to scientists for decades because normally the gas is neutral," he said. "So, what is supplying enough energy to knock the negatively charged electrons out of them?"
The central region of the Milky Way Galaxy is obscured by clouds of gas and dust, but infrared light and other strong emissions can get through for us to detect. Courtesy NASA-JPL/Caltech.
Cosmic rays seem to be one possibility, but the energy signatures in the zone aren't enough to explain them. So, the team turned to dark matter. They reviewed the influence of WIMPs. Particles of this stuff can pass through regular matter, and we wouldn't even know it. They're just really hard to detect. The team then proposed an interaction by particles with much lower mass than WIMPs. These tiny dark matter particles could be crashing into each other. The result is swarms of newly charged particles. That process is called "annihilation". The resulting charged particles would be energetic enough to ionize the CMS's hydrogen gas.
Using the CMZ as a Probe for Dark Matter
The team's idea provides another tool in the hunt for finding and understanding dark matter. “The search for dark matter is one of fundamental science’s most important objectives," said Balaji. "A lot of experiments are based on Earth, waiting with hands outstretched for the dark matter to come to them. By peering into the center of our Milky Way, the hydrogen gas in the CMZ is suggesting that we may be closer to identifying evidence on the possible nature of dark matter.”
Dark matter is known only by its effect on regular matter (so far). The best-known effect is gravitational. Everything with mass has a gravitational effect, and dark matter has the influence to shape a galaxy by keeping it "corralled" as it moves through space. Our own Milky Way has a dark matter halo that does this. In addition, dark matter keeps galaxy clusters together through gravitational influence. We also see its effect in gravitational lensing, and it's played a role in the evolution of structure in the Universe. So, there's a lot we know about the stuff.
Dark matter affects the path of light as it travels, distorting and bending its trajectory. Courtesy STScI.
However, that pales in comparison to our understanding of what this unseen dark matter IS. Scientists have suggested WIMPs as one possibility for this so-far invisible material. If the evidence radiating from the CMZ turns out to be a weaker form of WIMP (or even some new weak dark, low-mass particle), that would explain the emissions and really open up the search for all forms of dark matter.
For More Information
Mysterious Phenomenon at Centre of Galaxy Could Reveal New Kind of Dark Matter
Anomalous Ionization in the Central Molecular Zone by Sub-GeV Dark Matter
