Astronomers using the Hubble Space Telescope have discovered a new type of celestial object. Cloud 9 was a starless, gas-rich cloud of dark matter that was slightly too light to become a full-fledged galaxy.
The strange object is located near the spiral galaxy Messier 94 (M94), more than 14 million light-years from Earth, as detailed in a study published Nov. 10 in The Astrophysical Journal Letters and presented this week at the 247th Annual Meeting of the American Astronomical Society in Phoenix. Cloud-9 is a cosmic relic and a primordial building block of galaxies, supporting the critical mass threshold required for gas bodies and dark matter to collapse into galaxies.
As a result, the Cloud-9 discovery provides strong support for the basis of the Lambda Cold Dark Matter Model (LCDM), a major cosmological framework aimed at explaining the structure and composition of the Universe. One of the model’s main predictions is that dark matter may not become heavy enough to settle in the halo and anchor the galaxy.
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Deep Anand, an astronomer at the Space Telescope Science Institute (STScI) and lead author of the study, told Live Science in an email that “there must be many of these ‘dark halos’, but most of them do not hold hydrogen gas, so they remain invisible.” “Cloud-9 is at the upper end of the dark halo mass range, so it can hold gas and therefore be visible in radio observations. This is certainly strong support for the fundamental predictions of LCDM.”
Cloud-9 therefore provides the first hint of evidence that the universe may be filled with starless, low-mass dark matter halos, as theory predicts.
excavate space fossils
Astronomers discovered Cloud-9 three years ago using the 500-meter Aperture Spherical Telescope (FAST) in China’s Guizhou province. The giant radio telescope has been “very productive in finding similar clouds” and may find others in the future, study co-author and STScI astronomer Andrew Fox told Live Science in an email.
Researchers previously used the Very Large Array, a 28-telescope array in New Mexico, to zero in on the peak of Cloud 9’s radio emissions, which emanate from its 5,000 light-year-wide core. However, due to possible sensitivity limitations of the telescope, this observation was not able to determine the true nature of the object. Perhaps Cloud-9 was just a fuzzy dwarf galaxy, the researchers thought, too faint to be properly observed using ground-based facilities.
But as described in a new study, follow-up observations by the Hubble Space Telescope’s advanced survey camera have revealed a far more unusual phenomenon that astronomers have been searching for for years. It is a “theoretical phantom object” and the first confirmed RELHIC (reionization limit HI cloud) in history. In other words, it’s a cloud of neutral hydrogen, a remnant from the early universe’s birth and a unique “window into the dark universe,” Fox said in a NASA press statement.
This detection of hydrogen proved that Cloud-9 is not a typical dwarf galaxy, but something strange.
Is it a galaxy or not?
Researchers analyzed the gas within Cloud 9 based on the radio waves it emits and found that the gas gives this strange object a mass equal to about 1 million suns. That alone is not enough to sustain such a large cloud of gas. Therefore, assuming the system is maintained by a balance between gravity, gas pressure, and gas heating, the researchers calculated that Cloud-9’s dark matter component is equivalent to about 5 billion solar masses.
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This mass hits a sweet spot that is “remarkably close” to the uniquely theorized critical mass threshold. At this threshold, Cloud-9 is just shy of being massive enough to collapse into a galaxy, but it has enough mass to sustain itself due to its dark matter content.
Cloud-9 is also in thermal equilibrium with the cosmic ultraviolet (UV) background, the UV energy flowing from all the stars in the universe, an active black hole, and hot gas. This energy ionizes, or charges, the gas and keeps it relatively hot, inhibiting galaxy formation. This also contributes to the complete absence of stars in the cloud.
But researchers conclude that Cloud-9 may not be irrevocably doomed to eternal darkness. It may still collect enough mass to become a galaxy, but the exact dynamics that allow this to happen are still speculative.
Whatever its fate, Cloud-9 serves as a physical benchmark to show that current dark matter models and theories of galaxy formation are on the right track.
A very rare relic from ancient space
Future research will look for failed galaxies similar to Cloud-9, but finding them is easier said than done for a variety of reasons. First, such dim objects can easily outshine other objects.
These clouds are also temporary, likely disappearing through a process called ram pressure stripping that strips away gas as it travels through intergalactic space. In fact, Cloud-9 appears to have already been perturbed by the relatively hot perigalactic material surrounding neighboring galaxy M94, the researchers said.
“In order to survive into modern times as a gas-rich dark cloud, a system must meet two strict and statistically rare criteria,” Alejandro Benítez Lambay, principal investigator of the Cloud-9 research program and an astrophysicist at the University of Milano-Bicocca, told Live Science via email. “First, the assembly history of that dark matter halo must be unusually slow; if it grew too fast in the early Universe, the gas would have collapsed to form stars before it could be heated by the cosmic ultraviolet background. Second, the system must remain sufficiently isolated.” Benitez Lambay says that only 10% of such gas clouds remain starless and pristine, like Cloud-9. It added that it could be less than that.
Finally, as an ambassador of the dark universe, Cloud-9 serves as an important reminder that the stunning panoramas of stars we see in most astronomical images represent only a small portion of the entire universe, and that the shiny things we can see are only part of the cosmological story.
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