Hunt for dark matter narrows, US’ galactic simulations reach one step closer to decode mystery

A new study from the University of Washington is offering astronomers a more cautious — but potentially more powerful — way to search for dark matter. The research suggests that some of the strange features seen in stellar streams may not be caused by dark matter at all. Instead, the galaxy itself can create many of these features.The finding could help scientists distinguish genuine signs of dark matter from misleading signals. Dark matter connection Our Milky Way may look relatively orderly when viewed from afar, with most of its stars concentrated in a broad, rotating disc. But the region surrounding the galaxy is far more complicated. Among the most interesting structures found there are stellar streams — long, narrow trails of stars that orbit a galaxy.A stellar stream can form when a group of stars, such as a star cluster or a small satellite galaxy, becomes trapped by the gravity of a larger galaxy. Over time, the larger galaxy’s gravitational pull stretches the group of stars into an elongated stream.Astronomers are particularly interested in these streams because they behave like sensitive tracers of the gravitational environment around them. Even relatively small changes in gravity can leave visible signatures in their shapes.That has led scientists to consider stellar streams as potential detectors of dark matter. One leading idea has been that small concentrations of dark matter, known as subhalos, could disturb stellar streams as they travel through the Milky Way. If a dark-matter clump passed near a stellar stream, its gravity could potentially produce a gap, kink, bend or other disturbance in the stream.At first glance, this seems like an ideal method for finding something that cannot be observed directly: look for the gravitational fingerprints it leaves behind.But there is a problem. What if something other than dark matter can create the same fingerprints? That is the question researchers at the University of Washington set out to investigate. Dark matter makes up most of the mass in the universe “Dark matter makes up most of the mass in the universe and forms the scaffolding that galaxies grow on, but we still don’t know what it is,” said co-author Nora Shipp, a UW assistant professor of astronomy. “The Milky Way is one of the best laboratories we have for figuring that out, and stellar streams are one of the sharpest tools inside it.”The researchers then populated the galaxies with approximately 15,000 simulated stellar streams and allowed the virtual systems to evolve for five billion years.The results were striking. Even without dark-matter subhalos, the vast majority of the simulated streams developed irregular structures.The researchers observed bends, wiggles, kinks, gaps, branches, spurs and clumps. Some streams were even completely disrupted by the gravitational environment of their host galaxies.The new study was an effort to understand the role that the host galaxy — rather than the dark matter clumps within it — plays in shaping stellar streams. Astronomers simulated four Milky Way-sized galaxies without any dark matter clumps, then peppered them with roughly 15,000 stellar streams. After five billion simulated years, the team observed irregularities in nearly every stellar stream, according to a press release.“In our simulations, the host galaxies alone caused the same kinds of irregularities that we observe in real stellar streams,” said lead author Arpit Arora, a UW postdoctoral scholar in astronomy. “Now that we can predict what the host galaxy does on its own, we can start isolating the part that dark matter is responsible for.”The cause of the irregularities was the structure of the galaxies themselves. In each simulated galaxy, stars were spread somewhat unevenly across the disc, creating areas of greater and lesser density to mimic the composition of a real galaxy like ours. As the simulated streams of stars passed through denser regions of space, they were bent and torn by the irregular gravitational landscape.Arora expected the host galaxies to impart some irregularities on the streams, but the sheer number caught him off guard.“We found that almost all of the streams had some sort of structural variation,” Arora said. “So this idea that streams are naturally thin and smooth wasn’t really necessarily true.”Get the latest in engineering, tech, space & science - delivered daily to your inbox.Prabhat, an alumnus of the Indian Institute of Mass Communication, is a tech and defense journalist. While he enjoys writing on modern weapons and emerging tech, he has also reported on global politics and business. He has been previously associated with well-known media houses, including the International Business Times (Singapore Edition) and ANI.