Astronomers may have discovered a previously unknown type of astronomical object, nicknamed “Cloud-9,” that could shed light on dark matter, one of the biggest mysteries in the universe.

Dark matter is an enigmatic substance that shapes the cosmos and provides it with structure. While it has never been directly observed, dark matter is believed to make up 85% of the total matter in the universe and can be detected due to its gravitational effects.

Cloud-9 is thought to be a dark matter cloud that could be a remnant of galaxy formation from the early days of the universe, according to new research published Monday in The Astrophysical Journal Letters.

“This cloud is a window into the dark Universe,” study coauthor Andrew Fox, an astronomer at the Space Telescope Science Institute in Baltimore, said in a statement.

“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,” added Fox, who is affiliated with the European Space Agency (ESA) and the Association of Universities for Research in Astronomy (AURA).

It has been theorized that dark matter originated from the big bang that led to the creation of the universe 13.8 billion years ago and makes up cosmic clouds that never accumulated enough gas to form stars.

Recent observations made using the Hubble Space Telescope revealed Cloud-9 to be starless.

“This is a tale of a failed galaxy,” said study coauthor Alejandro Benitez-Llambay, astrophysicist and assistant professor in the physics department at the University of Milano-Bicocca 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.”

Future observations of the cloud, and the discovery of more objects like Cloud-9, could lead to a greater understanding of dark matter, galaxy formation and evolution, and the early days of the universe.

The search for phantom objects

Unlike bright stars or star-filled galaxies, failed galaxies are dark, which makes them difficult to find.

At first glance, Cloud-9 might have been mistaken for a faint dwarf galaxy, or a small galaxy composed of about 1,000 up to several billion stars, the research team said. Such galaxies are small when compared to our Milky Way, which contains hundreds of billions of stars.

Cloud-9 was first discovered three years ago during a survey of hydrogen gas near the Messier 94 galaxy by the Five-hundred-meter Aperture Spherical Telescope, or FAST, in China’s Guizhou province. The Green Bank Telescope and Very Large Array observatory were used for follow-up observations, and the latest Hubble findings confirmed the failed galaxy’s starless nature.

“Theories of galaxy formation predicted there is minimum threshold of dark matter required to ignite star formation and turn a dark cloud into a luminous galaxy,” Fox said. “With Cloud-9, we have an example of an object just below this threshold, containing no stars.”

The object was given its nickname because it is the ninth gas cloud found on the outskirts of the spiral galaxy Messier 94. Slight distortions of gas suggest there may be interactions between the cloud and galaxy.

While astronomers have observed hydrogen clouds before, Cloud-9 stands out as compact and spherical rather than being irregular in size.
But that doesn’t mean Cloud-9 isn’t large.

The core is composed of neutral hydrogen and spans 4,900 light-years in diameter. A light-year is the distance light travels in one year, which is 5.88 trillion miles (9.46 trillion kilometers). The amount of hydrogen within the cloud is about 1 million times the mass of the sun. Astronomers estimate that about 5 billion solar masses of dark matter make up the rest of the cloud.

“There must be a massive amount of ‘invisible’ gravity holding it together,” said study coauthor Dr. Rachel Beaton, assistant astronomer at the Space Telescope Science Institute. “The neutral hydrogen we see simply doesn’t provide enough mass; there must be a dark matter halo providing the gravitational support” and acting like invisible scaffolding for the cloud.

Currently, Cloud-9 exists in a state of fine balance, Beaton added.

“It has just enough mass to hold onto its gas, but not quite enough to force that gas into star formation,” she wrote in an email “This rarity explains why we haven’t found many of these objects in the local universe — most halos either lose their gas entirely or become full-fledged galaxies.”

Defining Cloud-9’s true nature

Cloud-9 still could turn into a galaxy, the team noted. If the object accumulates more mass, the gas within the cloud will collapse and fragment, leading to the formation of stars — which would transform Cloud-9 into a late bloomer galaxy.

“However, the other possibility is that the cloud loses mass, which could happen if it falls closer in toward M94,” Fox said, referring to Messier 94, “and its gas gets stripped like a cloud in a wind tunnel until the cloud ceases to exist.”

Future high-resolution observations could provide a clearer look at Cloud-9’s core, a “holy grail” that could reveal how much dark matter is contained in the object’s center.

“We can rule out certain dark matter candidates — a major step to constrain the nature of the dark matter particle itself using astrophysical data,” Beaton said.

While Cloud-9 is an intriguing cloud, it’s not the first to be discussed within scenarios of “dark galaxies,” and more mundane explanations can’t be excluded, said Dr. Jacco van Loon, an associate professor of astrophysics and director of the Keele Observatory at Keele University in England.

For example, another hydrogen cloud noted in the study, called FAST J0139+4328, was recently found to be a very faint galaxy, van Loon said.

“The mass in stars found to belong to FAST J0139+4328 is ten times higher than what they thought could be possible, so it is conceivable that in their own cloud, which contains almost a hundred times less hydrogen than FAST J0139+4328, there resides an even fainter galaxy than what they could have expected to detect even with the Hubble Space Telescope,” he wrote in an email. “One might speculate, but to claim an optically-dark gas cloud is a dark-matter relic requires stronger, unambiguous evidence.”

Van Loon was not involved in the research but reached out to the team and expressed caution about the assertions of Cloud-9 being a dark matter cloud when he saw the announcement of the research prior to its publication. He is mentioned in the study’s acknowledgements.

In the meantime, the team continues to search for similar objects to determine whether Cloud-9 is an oddball or part of a larger cohort of relics.

The latest Hubble observations add an important piece to the puzzle of this intriguing object, said Dr. Kristine Spekkens, professor of astronomy, astrophysics and relativity at Queen’s University in Kingston, Ontario. Spekkens was not involved in the new study, but she previously observed Cloud-9 using the Green Bank Telescope.

“Further study of this starless gas, such as its unusual shape, will help unravel its origin as a starless galaxy or a free-floating cloud,” Spekkens said. “Whatever its nature, Cloud-9 is a fantastic example of the bright future of this field of study in helping researchers unlock the mysteries of the universe.”

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