Galaxies typically shine in a number of regions of the spectrum. Credit: infrared: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Ghent; X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, MPE; optical: R. Gendler.
Galaxies can come in a variety of shapes and sizes, but some of their properties are pretty consistent. They usually have a supermassive black hole at their core, are made up of stars with cosmic gas and dust, and rest in the arms of dark matter that appears to help hold galaxies together. Dark matter is indeed dark, but the other components of galaxies usually glow more or less, sometimes really brilliantly if they are heated to high temperatures.
As a result, galaxies are like dazzlingly bright islands in the black ocean of space. But as it turns out, there are also very strange exceptions. One of them is a remarkable dwarf galaxy located 94 million light-years away, making it one of the closest galaxies in the observable universe. It was named FAST J0139+4328 and does not shine at all in the visible region of the spectrum. In fact, it hardly shines in any part of the spectrum.
FAST J0139+4328 appears to be what has recently been called a “dark galaxy.” It appears to be made entirely of dark matter except for a smattering of stars. The galaxy was discovered by a team of Chinese experts who then discovered that it is a dark dwarf galaxy, which is quite isolated. According to them, this is the first time that an isolated dark galaxy has been discovered in the nearby universe.
While the dark matter concept nicely explains the problems with galaxy rotation where observations don’t match predictions that don’t account for dark matter, that doesn’t mean it’s without its own conflicts. One of them is the problem with dwarf galaxies, which is that there are few, far fewer than the dark matter universe simulations make.
Radio telescope FAST. Credit: Absolute Cosmos.
One possible explanation is that we are unable to detect the abundance of dark dwarf galaxies because they are almost invisible. Moreover, from our point of view, they often merge with the surrounding galaxies. But when it comes to isolated galaxies, our chances of detecting them are greater.
Dwarf spiral galaxy NGC 5474. Credit: NASA.
Xu and his colleagues used the recently launched radio telescope with the world’s largest single antenna, China’s FAST. It was actually a side project when scientists took advantage of the empty “windows” in this radio telescope’s observing program. They searched the large clouds of neutral hydrogen in intergalactic space for clues of the presence of galaxies. In doing so, they encountered the mentioned dark galaxy FAST J0139+4328.
Their observations in the radio region corresponded to a small galaxy in the shape of a rotating disc, which is not visible at all in the visible radiation region. According to the calculations of the Chinese team, this dark galaxy contains stars with a mass of about 690 thousand Suns, together with cosmic gas, mostly neutral hydrogen, with a mass of 83 million Suns. The combined mass of ordinary matter in this galaxy is about 110 million Suns.
But this mass is only an absolute fraction of the total mass of the galaxy FAST J0139+4328. According to calculations derived from the rotation speed of this dwarf galaxy, it should weigh a total of 5.1 billion Suns. If the Chinese are right, that means this galaxy is about 98 percent dark matter. For experts, it is extremely attractive, because it could represent the hitherto practically unknown earliest stage of galaxy evolution. Her research will definitely continue.
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