NASA’s Hubble Examines Cloud-9, First of New Type of Object

A region of space mostly filled with background galaxies, with one prominent star at upper left. A large blob of purple haze occupies much of the field. Within the purple region, an unremarkable area is outlined with a dashed white circle. — This image shows the location of Cloud-9, which is 14 million light-years from Earth. The diffuse magenta is radio data from the ground-based Very Large Array (VLA) showing the presence of the cloud. The dashed circle marks the peak of radio emission, which is where researchers focused their search for stars. Follow-up observations by the Hubble Space Telescope’s Advanced Camera for Surveys found no stars within the cloud. The few objects that appear within its boundaries are background galaxies. Before the Hubble observations, scientists could argue that Cloud-9 is a faint dwarf galaxy whose stars could not be seen with ground-based telescopes due to the lack of sensitivity. Hubble’s Advanced Camera for Surveys shows that, in reality, the failed galaxy contains no stars. Science: NASA, ESA, VLA, Gagandeep Anand (STScI), Alejandro Benitez-Llambay (University of Milano-Bicocca); Image Processing: Joseph DePasquale (STScI)

A team using NASA’s Hubble Space Telescope has uncovered a new type of astronomical object — a starless, gas-rich, dark-matter cloud considered a “relic” or remnant of early galaxy formation. Nicknamed “Cloud-9,” this is the first confirmed detection of such an object in the universe — a finding that furthers the understanding of galaxy formation, the early universe, and the nature of dark matter itself.

“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.”

The results, published in The Astrophysical Journal Letters, were presented at a press conference Monday at the 247th meeting of the American Astronomical Society in Phoenix.

“This cloud is a window into the dark universe,” said team member Andrew Fox of the Association of Universities for Research in Astronomy/Space Telescope Science Institute (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.”

The object is called a Reionization-Limited H I Cloud, or “RELHIC.” The term “H I” refers to neutral hydrogen, and “RELHIC” describes a natal hydrogen cloud from the universe’s early days, a fossil leftover that has not formed stars. For years, scientists have looked for evidence of such a theoretical phantom object. It wasn’t until they turned Hubble toward the cloud, confirming that it is indeed starless, that they found support for the theory.

“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,” said lead author Gagandeep Anand of STScI. “But with Hubble’s Advanced Camera for Surveys, we’re able to nail down that there’s nothing there.”

The discovery of this relic cloud was a surprise. “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.

Astronomers think RELHICs are dark matter clouds that couldn’t 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 insights into the dark components of the universe that are difficult to study through traditional observations, which focus 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 bigger and more irregular than Cloud-9. Compared with other observed hydrogen clouds, Cloud-9 is smaller, more compact, and highly spherical, making it look very different from the others.

The core of this object is composed of neutral hydrogen and is about 4,900 light-years in diameter. Researchers measured the hydrogen gas in Cloud-9 by the radio waves it emits, measuring it to be approximately one million times the mass of the Sun. Assuming that the gas pressure is balancing the dark matter cloud’s gravity, which appears to be the case, researchers calculated Cloud-9’s dark matter must be about five billion solar masses.

Cloud-9 is an example of structures and mysteries that don’t involve stars. Just looking at stars doesn’t give the full picture. Studying the gas and dark matter helps provide a more complete understanding of what’s going on in these systems that would otherwise be unknown.

Read more on the NASA science website (original source)