NASA’s Hubble Examines Cloud-9, First of New Type of Object
A team using NASA’s Hubble Space Telescope has discovered a new type of astronomical object: a starless, gas-rich cloud of dark matter believed to be a “relic” or remnant of early galaxy formation. Dubbed “Cloud-9,” it is the first confirmed detection of such an object in the universe – a discovery that advances understanding of galaxy formation, the early universe and the nature of dark matter itself.
“It’s the story of a bankrupt galaxy,” said the program’s principal investigator, Alejandro Benitez-Llambay of Milano-Bicocca University in Milan, Italy. “In science, we generally learn more from failures than from successes. In this case, not seeing stars is what proves the theory right. This tells us that we have found in the local universe a primordial building block of a galaxy that did not form.”
The results, published in The Astrophysical Journal Letters, were presented Monday at a news conference at the 247th meeting of the American Astronomical Society in Phoenix.
“This cloud is a window into the dark universe,” said Andrew Fox, a member of the Association of Universities for Research in Astronomy/Space Telescope Science Institute (AURA/STScI) team for the European Space Agency. “We know theoretically that most of the mass in the universe should be dark matter, but it is difficult to detect this dark matter because it does not emit light. Cloud-9 gives us a rare glimpse of a cloud dominated by dark matter.”
This image shows the location of Cloud-9, which is 14 million light years from Earth. The diffuse magenta is made up of radio data from the Very Large Array (VLA) on the ground showing the presence of the cloud. The dotted circle marks the peak of the radio emission, where the researchers focused their search for stars. Follow-up observations by the Hubble Space Telescope’s Advanced Camera for Surveys found no stars in the cloud. The few objects that appear within its boundaries are background galaxies. Before Hubble’s observations, scientists could say that Cloud-9 was a pale dwarf galaxy whose stars could not be seen with ground-based telescopes due to lack of sensitivity. Hubble’s Advanced Camera for Surveys shows that in reality, the bankrupt galaxy contains no stars.
Sciences: NASA, ESA, VLA, Gagandeep Anand (STScI), Alejandro Benitez-Llambay (University of Milan-Bicocca); Image processing: Joseph DePasquale (STScI)
The object is called Reionization-Limited HI Cloud, or “RELHIC”. The term “HI” refers to neutral hydrogen, and “RELHIC” describes a cloud of natal hydrogen from the early days of the universe, a fossil remnant that did not form stars. For years, scientists have searched for evidence of the existence of such a theoretical ghost object. It wasn’t until they turned Hubble toward the cloud, confirming that it was indeed starless, that they found support for their theory.
“Before using Hubble, we could say that this was a faint dwarf galaxy that we couldn’t see with ground-based telescopes. They just didn’t achieve deep enough sensitivity to discover stars,” said lead author Gagandeep Anand of STScI. “But with Hubble’s advanced survey camera, we are able to determine that there is nothing there.”
The discovery of this relic cloud was a surprise. “Among our galactic neighbors there might be a few abandoned houses,” said STScI’s Rachael Beaton, who is also part of the research team.
Astronomers think RELHICs are clouds of dark matter that cannot accumulate enough gas to form stars. They represent a window into the early stages of galaxy formation. Cloud-9 suggests the existence of many other small dark matter-dominated structures in the universe – other failed galaxies. This discovery provides new information on the dark components of the universe, difficult to study through traditional observations, focused on bright objects like stars and galaxies.
Scientists have studied hydrogen clouds near the Milky Way for many years, but these clouds tend to be much larger and more irregular than Cloud-9. Compared to other hydrogen clouds observed, Cloud-9 is smaller, more compact and highly spherical, making it very different from the others.
The core of this object is composed of neutral hydrogen and is approximately 4,900 light years across. Researchers measured the hydrogen gas in Cloud-9 using the radio waves it emits, measuring it at about a million times the mass of the Sun. Assuming that the gas pressure balances the gravity of the dark matter cloud, which appears to be the case, the researchers calculated that the dark matter in Cloud-9 must represent about five billion solar masses.
Cloud-9 is an example of structures and mysteries that don’t involve stars. Just looking at the stars doesn’t give the complete picture. Studying gas and dark matter provides insight into what is happening in these systems that would otherwise be unknown.
From an observational point of view, identifying these failed galaxies is challenging because nearby objects eclipse them. Such systems are also vulnerable to environmental effects such as dynamic pressure extraction, which can remove gas as the cloud moves through intergalactic space. These factors further reduce their expected numbers.
The starless relic was discovered three years ago as part of a radio survey by the Five Hundred Meter Aperture Spherical Telescope (FAST) in Guizhou, China, a discovery later confirmed by the Green Bank Telescope and Very Large Array facilities in the United States. But it was only with Hubble that researchers were able to determine with certainty that the bankrupt galaxy does not contain any stars.
Cloud-9 was simply named sequentially, having been the ninth gas cloud identified on the outskirts of a nearby spiral galaxy, Messier 94 (M94). The cloud is close to M94 and appears to have a physical association with the galaxy. The high-resolution radio data shows slight distortions of the gas, possibly indicating an interaction between the cloud and the galaxy.
The cloud could possibly form a galaxy in the future, provided it becomes more massive – although how this would happen remains speculative. If it were much larger, say, more than 5 billion times the mass of our Sun, it would have collapsed, formed stars, and become a galaxy that would be no different from any other galaxy we see. If it were much smaller than that, the gas could have been dispersed and ionized and there wouldn’t be much left. But he’s in a sweet spot where he could stay as a RELHIC.
The absence of stars in this object provides a unique window into the intrinsic properties of dark matter clouds. The rarity of such objects and the potential for future studies should encourage the discovery of more of these “failed galaxies” or “relicts”, leading to a better understanding of the early universe and the physics of dark matter.
The Hubble Space Telescope has been operating for more than three decades and continues to make groundbreaking discoveries that shape our fundamental understanding of the universe. Hubble is an international cooperation project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Astronomical Research, conducts Hubble science operations for NASA.
