In a fascinating new study published in Science Advances, astronomers have discovered a peculiar, fast-moving cloud of gas near the Milky Way that may not be a cloud at all. Instead, it could be a small galaxy composed primarily of dark matter. This finding opens up new possibilities for understanding the mysterious substance that makes up the majority of matter in the universe but is invisible to telescopes.
The Mystery of AC G185.0-11.5
The object in question, known as AC G185.0-11.5, is part of a larger group of gas clouds called the AC-I Complex. These clouds are known as high-velocity clouds (HVCs), which are moving much faster than the gas in the plane of the Milky Way, sometimes reaching speeds of several hundred miles per hour. It’s believed that these HVCs may be material falling onto our galaxy from external sources, like gas from torn-apart galaxies or even remnants of supernovae that were ejected into space.
However, as researchers from the Chinese National Academy of Sciences studied AC G185.0-11.5 using data from the Arecibo Observatory (before its collapse) and the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in China, they discovered something unexpected. The object appeared to be a rotating disk-like structure, very much resembling a galaxy, yet with one key difference: it contained no visible stars.
This unusual property led the scientists to hypothesize that AC G185.0-11.5 could be a dark galaxy—a type of galaxy that contains very few stars but is dominated by dark matter. Dark galaxies are particularly elusive because their lack of visible stars makes them difficult to detect, but astronomers believe that they contain significant amounts of dark matter, which doesn’t interact with light and can only be detected through its gravitational effects.
What Makes a Dark Galaxy?
The researchers found that AC G185.0-11.5 is rotating in a way that suggests the presence of dark matter within the galaxy. Ming Zhu, an astrophysicist involved in the study, explains that the way the gas is rotating in a disk-like structure indicates that dark matter must be present to exert the gravitational forces required to control that rotation.
In the study, the team calculates that AC G185.0-11.5 has a mass equivalent to 300 million times the mass of the Sun, although there is still some uncertainty in this estimate. They also determined that the dark galaxy is located approximately 900,000 light-years from the Milky Way, although, given the uncertainties in mass, the distance measurement is still tentative.
While dark galaxies are theorized to exist, few have been identified, and many candidate dark galaxies have been found to be ordinary gas clouds with unusual interactions. However, the researchers are hopeful that AC G185.0-11.5 is indeed a dark galaxy, due to the unique properties observed in its rotation and the lack of stars.
Why Is This Discovery Important?
If AC G185.0-11.5 is confirmed to be a dark galaxy, its proximity to the Milky Way could provide a valuable opportunity to study the behavior and nature of dark matter more directly. Dark matter makes up around 85% of the mass in the universe, but it doesn’t interact with light, which makes it nearly impossible to observe directly. Instead, its existence has been inferred based on its gravitational effects on visible matter.
For decades, astronomers have known that dark matter affects the way galaxies rotate. By comparing the expected rotation of galaxies based on visible matter alone with the actual rotation speeds, scientists concluded that there must be additional unseen mass—dark matter—pulling on the galaxies. A dark galaxy like AC G185.0-11.5, which is believed to be primarily composed of dark matter, could provide crucial insights into how dark matter behaves and influences the formation and rotation of galaxies.
The Road Ahead: Studying AC G185.0-11.5
While the discovery of AC G185.0-11.5 as a potential dark galaxy is groundbreaking, it’s important to note that the results still require further confirmation. Astronomical observations and measurements are often fraught with uncertainty, especially when dealing with objects as distant and mysterious as high-velocity clouds. But the proximity of AC G185.0-11.5 to the Milky Way could allow astronomers to study it more easily with both space-based and ground-based telescopes.
In the future, observatories like the James Webb Space Telescope (JWST) could potentially detect any stars that might be present within the galaxy, while the Very Large Array could help create detailed images of the galaxy’s rotating disk and reveal more about its structure and composition.
Conclusion
The discovery of AC G185.0-11.5 opens up exciting new possibilities in the study of dark matter and galaxy formation. If further observations confirm that it is indeed a dark galaxy, this could help scientists unlock some of the mysteries surrounding the elusive substance that makes up most of the universe’s mass. As astronomers continue to explore the cosmos, this finding could provide a rare opportunity to study a galaxy dominated by dark matter, offering new insights into the very nature of the universe itself.
