Hubble Telescope finds dark-matter cloud

A team using NASA’s Hubble Space Telescope found a new type of astronomical object, a starless, gas-rich, dark-matter cloud considered a remnant of early galaxy formation. The object, nicknamed “Cloud-9,” is the first confirmed detection of such an object in the universe. 

“This is a tale of a failed galaxy,” said the program’s principal investigator, Alejandro Benitez-Llambay of the Milano-Bicocca University in Milan, Italy. “In science, we usually learn more from the failures than from the successes. In this case, seeing no stars is what proves the theory right. It tells us that we have found in the local universe a primordial building block of a galaxy that hasn’t formed.”

The object is called a Reionization-Limited H I Cloud, or “RELHIC.” The “H I” refers to a neutral hydrogen cloud from the universe’s early days that never formed stars. Scientists have been looking for a theoretical object like this for years, only finding support for this theory with the starless Cloud-9. 

“This cloud is a window into the dark universe,” said team member Andrew Fox of the Association of Universities for Research in Astronomy/Space Telescope Science Institute (AURA/STScI) for the European Space Agency. “We know from theory that most of the mass in the universe is expected to be dark matter, but it’s difficult to detect this dark material because it doesn’t emit light. Cloud-9 gives us a rare look at a dark-matter-dominated cloud.”

This discovery suggests the existence of other small, dark matter-dominated structures. Scientists have studied hydrogen clouds near the Milky Way for many years, but they tend to be much bigger and more irregular than Cloud-9. Compared with other hydrogen clouds, Cloud-9 is smaller, more compact and spherical. 

Cloud-9’s core is made of neutral hydrogen and is about 4,900 light-years in diameter. Researchers measured the hydrogen in the cloud by the radio waves it emits, estimating it at approximately one million times the mass of the Sun. They calculated that the dark matter in the cloud is about five billion solar masses. 

Identifying failed galaxies like Cloud-9 is difficult because nearby objects outshine them. These systems are also vulnerable to environmental effects like ram-pressure stripping, which removes gas as the cloud moves through space. 

Cloud-9 was discovered three years ago as part of a radio survey by the Five-hundred-meter Aperture Spherical Telescope (FAST) in Guizhou, China. Only with the Hubble, however, could researchers definitively see that the failed galaxy did not contain stars. It was named sequentially as the ninth gas cloud identified on the outskirts of a nearby spiral galaxy called Messier 94.