Chama-se Cloud-9
Astronomers may have discovered a previously unknown type of astronomical object, dubbed “Cloud-9,” that could shed light on dark matter, one of the Universe’s greatest mysteries.
Dark matter is an enigmatic substance that shapes the cosmos and gives it structure. Although 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 considered a cloud of dark matter that may be a remnant of the formation of galaxies in the early Universe, according to new research published in Astrophysical Journal Letters.
“This cloud is a window into the dark Universe,” says study co-author Andrew Fox, an astronomer at the Space Telescope Science Institute in Baltimore, in a statement.
“We know from theory that most of the mass in the Universe must be dark matter, but this dark matter is difficult to detect because it does not emit light. Cloud-9 gives us a rare view of a cloud dominated by dark matter,” adds Fox, who is linked to the European Space Agency (ESA) and the Association of Universities for Research in Astronomy (AURA).
It has been theorized that dark matter originated in the Big Bang, which led to the creation of the Universe 13.8 billion years ago, and constitutes cosmic clouds that never accumulated enough gas to form stars.
Recent observations made with the Hubble Space Telescope revealed that Cloud-9 has no stars.
“This is the tale of a failed galaxy,” says study co-author Alejandro Benitez-Llambay, an astrophysicist and assistant professor in the physics department at the University of Milano-Bicocca in Milan, Italy. “In science, we usually learn more from failures than from successes. In this case, not seeing stars is what proves the theory is correct. It tells us that we have found in the local universe a primordial building block of a galaxy that has not yet formed.”
Future observations of the cloud, and the discovery of more objects like Cloud-9, could lead to a greater understanding of dark matter, the formation and evolution of galaxies, and the beginnings of the Universe.
Searching for ghost objects
Unlike bright stars or star-filled galaxies, failed galaxies are dark, which makes them difficult to find.
At first glance, Cloud-9 could have been mistaken for a faint dwarf galaxy or a small galaxy made up of about 1,000 to several billion stars, the research team says. These galaxies are small compared to our Milky Way, which contains hundreds of billions of stars.
Cloud-9 was discovered three years ago during a study 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 the Very Large Array observatory were used for follow-up observations and Hubble’s latest findings confirmed the starless nature of the failed galaxy.
“Theories of galaxy formation have predicted that there is a minimum threshold of dark matter necessary to trigger star formation and transform a dark cloud into a luminous galaxy,” says Fox. “With Cloud-9, we have an example of an object just below this threshold, which does not contain stars.”
The object was given the designation Cloud-9 because it is the ninth gas cloud found on the outskirts of the spiral galaxy Messier 94. Slight distortions of the gas suggest there may be interactions between the cloud and the galaxy.
Although astronomers have observed hydrogen clouds before, Cloud-9 stands out because it is compact and spherical, rather than irregular in size. But that doesn’t mean Cloud-9 isn’t big.
The core is composed of neutral hydrogen and has a diameter of 4,900 light years. A light year is the distance that light travels in a year, which is 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 an enormous amount of ‘invisible’ gravity holding it together,” says study co-author Rachel Beaton, assistant astronomer at the Space Telescope Science Institute. “The neutral hydrogen we see simply doesn’t provide enough mass; there has to be a halo of dark matter that provides the gravitational support” and acts as an invisible scaffold for the cloud.
Currently, Cloud-9 is in a state of balance, adds Beaton.
“It has just enough mass to hold the gas, but not enough to force that gas to form stars,” he writes in an email. “This rarity explains why we haven’t found many of these objects in the local universe — most halos either completely lose their gas or become full-fledged galaxies.”
Define the true nature of Cloud-9
Cloud-9 could still turn into a galaxy, the team notes. If the object accumulates more mass, the gas inside the cloud will collapse and fragment, leading to star formation — which would transform Cloud-9 into a late-blooming galaxy.
“However, the other possibility is that the cloud loses mass, which could happen if it falls closer to M94,” says Fox, referring to Messier 94, “and is stripped of its gas like a cloud in a wind tunnel until the cloud ceases to exist.”
Future high-resolution observations could give a clearer view of Cloud-9’s core, a “Holy Grail” that could reveal how much dark matter is contained at the object’s center.
“We can rule out certain dark matter candidates — an important step toward constraining the nature of the dark matter particle itself using astrophysical data,” says Beaton.
While Cloud-9 is an intriguing cloud, it is not the first to be discussed in “dark galaxy” scenarios – and more mundane explanations cannot be ruled out, says Jacco van Loon, associate professor of astrophysics and director of the Keele Observatory at the University of Keele in England.
For example, another hydrogen cloud mentioned in the study, called FAST J0139+4328, was recently discovered to be a very faint galaxy, van Loon points out.
“The mass of the stars discovered to belong to FAST J0139+4328 is ten times greater than they thought possible, so it is conceivable that in their own cloud, which contains almost a hundred times less hydrogen than FAST J0139+4328, there is a galaxy even fainter than the one they expected to detect even with the Hubble Space Telescope,” he writes in an email. “One can speculate, but to assert that an optically dark gas cloud is a relic of dark matter requires stronger and more unequivocal evidence.”
Van Loon was not involved in the investigation, but contacted the team and expressed caution about claims that Cloud-9 was a dark matter cloud when he saw the research announcement before its publication. It is mentioned in the study acknowledgments.
Meanwhile, the team continues to search for similar objects to determine whether Cloud-9 is an isolated case or part of a larger group of relics.
Hubble’s latest observations add an important piece to the puzzle of this intriguing object, said Kristine Spekkens, professor of astronomy, astrophysics and relativity at Queen’s University in Kingston, Ontario. Spekkens was not involved in the new study, but previously observed Cloud-9 using the Green Bank Telescope.
“Further study of this starless gas, as well as its unusual shape, will help unravel its origin as a starless galaxy or floating cloud,” says Spekkens. “Whatever its nature, Cloud-9 is a fantastic example of the bright future of this field of study, helping researchers unlock the mysteries of the universe.”
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