Over the years, many astronomers have had the opportunity to celebrate the discovery of a new galaxy. Elena D’Onghia was recently part of an international team of astronomers who identified what may be one of the best candidates for a failed galaxy.
Using images and data obtained by NASA’s Hubble Space Telescope, D’Onghia, a professor of astronomy, discovered what’s known as a dark matter halo containing neutral hydrogen gas but no detectable stars.
“It’s something that has been predicted from the current paradigm of galaxy formation, but we never observed it before, so we didn’t know if these systems exist,” says D’Onghia. “Now we have a confirmation that they do.”
Located about 14 million light-years from Earth near the spiral galaxy M94, the object earned its nickname Cloud-9, from its initial appearance as a massive blob of coherent hydrogen gas. But unlike other gas clouds astronomers have encountered, deeper examination revealed no stars at all.
“It’s a system embedded in a dark matter halo,” says D’Onghia. “So, it’s one of the clearest candidates for a dark halo that contains gas but has not formed stars. It’s a failed galaxy.”
Astronomers have a technical term for these predicted dark matter halos sans stars: Reionization-Limited H I Clouds (RELHICs). D’Onghia likens discovering one to finding a fossil from the era when the first galaxies were assembling.
The international team behind the discovery also includes Alejandro Benítez-Llambay (University of Milano-Bicocca), Gagandeep Anand, Rachael Beaton and Andrew Fox (Space Telescope Science Institute) and Julio Navarro (University of Victoria).
D’Onghia suggests that further exploration of Cloud-9 could give researchers hints about the primordial ingredients for galaxy formation — and it likely won’t be the last failed galaxy that astronomers discover.
“The conditions of the early universe likely prevented star formation in systems like this,” she explains. “It’s telling us about the initial setup for galaxies and giving us insight into how a dark matter halo arranges itself before the star formation happens, and in which conditions the star formation is suppressed.”
If confirmed as a RELHIC, Cloud-9 would provide strong support for a key prediction of what’s known as the Cold Dark Matter model, a theory that many small dark matter halos exist in the universe, even if they never became luminous galaxies. Detecting such objects has been one of the long-standing challenges in cosmology.
“Discoveries like Cloud-9 remind us that the universe still holds surprises,” D’Onghia explains. “Sometimes the most exciting objects are not the brightest ones — but the ones that never managed to shine.”