Hubble finds a galaxy that never was

The discovery is detailed in a study published in The Astrophysical Journal Letters. Nicknamed Cloud 9, the object is a dense cloud of gas dominated by dark matter and contains no stars. It is the first confirmed example of what scientists call a reionization limited hydrogen cloud, a relic from the early Universe that never became a galaxy.

Cloud 9 lies near the spiral galaxy Messier 94 and appears to be physically linked to it. Radio observations show that the cloud shares the same motion through space as M94, placing it about 4.4 million parsecs from Earth. Despite its proximity in cosmic terms, Cloud 9 remained hidden because it emits almost no visible light.

“This is a tale of a failed galaxy. 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,” said the program’s principal investigator, Alejandro Benitez-Llambay of the Milano-Bicocca University in Milan, Italy.

Cloud 9 was first detected 3 years ago in a radio survey by the Five hundred meter Aperture Spherical Telescope in China and later confirmed by observatories in the United States. Radio data revealed neutral hydrogen gas, but only deep imaging with Hubble could settle the critical question of whether stars were present.

Using Hubble’s Advanced Camera for Surveys, the team searched for even the faintest stellar population. None was found. The observations rule out the presence of a dwarf galaxy with more than a few thousand solar masses in stars, making Cloud 9 effectively devoid of starlight.

“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. But with Hubble’s Advanced Camera for Surveys, we’re able to nail down that there’s nothing there,” explained lead author Gagandeep Anand of the Space Telescope Science Institute (STScI), Baltimore, USA.

The cloud is composed mainly of neutral hydrogen gas and is about 4900 light years across. The gas itself has a mass roughly one million times that of the Sun. To remain stable, however, the cloud must be embedded in a much larger halo of dark matter. Researchers estimate the total mass to be around 5 billion solar masses, placing Cloud 9 right at the threshold where galaxy formation is expected to fail.

“This cloud is a window into the dark Universe. 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,” explained team member Andrew Fox of AURA/STScI for the European Space Agency.

Current cosmological models predict that many dark matter structures never grow massive enough to form galaxies. Until now, firm observational proof of such objects was missing. Cloud 9 provides strong evidence that these predictions are correct.

Its compact and nearly spherical shape also sets it apart from other hydrogen clouds near the Milky Way, which are typically larger and more irregular. The discovery suggests that many more such relics may exist, though they are difficult to detect because nearby bright galaxies easily outshine them.

Earlier, Qazinform News Agency reported that Kazakhstan plans to expand its international telescope network with new installations in Spain and Uzbekistan.