Astronomers find a ghost galaxy made of dark matter
Astronomers working with the Hubble Space Telescope have identified an entirely new type of cosmic object. It is a cloud rich in gas and dominated by dark matter, yet it contains no stars. Scientists consider it a relic left behind from the earliest stages of galaxy formation. The object, known as “Cloud-9,” is the first confirmed example of its kind ever observed 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.”
A Rare Glimpse of the Dark Universe
“This cloud is a window into the dark Universe,” explained team member Andrew Fox of 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.”
Cloud-9 belongs to a category known as Reionization-Limited H I Clouds, or “RELHICs.” The term “H I” refers to neutral hydrogen, while “RELHIC” describes a hydrogen cloud formed in the early Universe that never progressed to form stars. Scientists had predicted the existence of such objects for years, but direct confirmation remained elusive. Only after observing Cloud-9 with Hubble were researchers able to verify that it truly contains no stars.
Ruling Out a Hidden Dwarf Galaxy
“Before we used Hubble, you could argue that this is a faint dwarf galaxy that we could not see with ground-based telescopes. They just didn’t go deep enough in sensitivity to uncover stars,” explained lead author Gagandeep Anand of the Space Telescope Science Institute (STScI), Baltimore, USA. “But with Hubble’s Advanced Camera for Surveys, we’re able to nail down that there’s nothing there.”
The discovery itself caught researchers off guard. “Among our galactic neighbors, there might be a few abandoned houses out there,” said STScI’s Rachael Beaton, who is also on the research team.
RELHICs are believed to be dark matter structures that never gathered enough gas to trigger star formation. Because of this, they preserve conditions from the early Universe. Cloud-9 points to the likely existence of many other small, dark matter-dominated objects, often described as failed galaxies. Studying them offers new insight into parts of the Universe that remain difficult to observe because most telescopes focus on bright stars and galaxies.
Source: science daily
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Astronomers find a ghost galaxy made of dark matter