South Africa’s MeerKAT radio telescope array has provided the data for yet another major astronomical discovery. This time it has enabled an international team of astronomers to discover an unprecedentedly large series of clouds of elemental hydrogen gas, without any associated stars.

MeerKAT, located in the Karoo region, roughly west of the Northern Cape town of Carnarvon, has won enthusiastic plaudits from the global astronomy and astrophysics communities because of the unprecedentedly high resolution data it is providing. This has already led to a number of significant scientific breakthroughs, despite the fact that MeerKAT was formally inaugurated only some three-and-a-half years ago.

Currently, MeerKAT is composed of 64 dishes, but the now-under-way MeerKAT extension project will add another 20 dishes. This increase will allow the maximum distance between the array’s dishes to be dramatically increased from the current 8 km to about 20 km. This will most significantly increase its sensitivity and the resolution of the radio images that it will be able to capture.

The new discovered string of ‘dark’ (cool) hydrogen clouds lies at the edge of a group of galaxies. This group is “relatively massive”, in the words of the South African Radio Astronomy Observatory press release.  The research team, led by Professor Gyula Józsa, Prof Michelle Cluver and Prof Thomas Jarrett, currently has two preliminary hypotheses about the gas clouds. One is that is they are composed of gas stripped from the member galaxies of the group. The other is that the hydrogen gas is primordial and drawn into the galactic group along a cosmic filament pathway, by gravity.

“Cosmic filaments are the highways along which mass concentrations come together under the action of gravity,” explains Cluver (Associate Professor at Swinburne University of Technology, in Melbourne, Australia). “We expect gas-rich galaxies to be associated with these structures, using their neutral hydrogen as fuel for star formation and growth. We therefore designed our blind shallow survey to search for this type of gas along such a filament. And indeed we found gas in many galaxies, but we did not expect anything like these clouds. They form a huge complex of tenuous atomic hydrogen gas that stretches over a distance of 1.3-million light years. Seven spots of concentrated gas can be discerned from the complex.”

The absence of stars within this string of hydrogen gas clouds is the puzzling fact. The mass of the gas is huge, being ten billion times the mass of our Sun. Such clouds of hydrogen are usually associated with particular galaxies, for which they act as gas reservoirs to feed the star formation process, growing the galaxy over incredibly long periods of time. Yet this does not appear to be the case with the newly discovered series of cool hydrogen clouds. These should not be able to exist on their own, as radiation from the surrounding galaxies would heat the gas up. It would require the gravitational pull of a galaxy to cause the gas cloud to concentrate and become dense enough to be shielded against the effects of radiation.

“We were very surprised that we did not find any indication of a significant amount of stars, despite our thorough search using very deep ultraviolet, optical and infrared imaging,” reported Jarrett (Astrophysics Professor at the University of Cape Town). “There have to be stars, it’s inconceivable for a cloud the size of a galaxy to simple be floating in space!”

“Only a handful of cloud complexes with similarities to this one are known and our new discovery seems to differ in quite some key aspects,” highlighted Józsa (of the Max-Planck-Institute for Radio Astronomy, in Germany). “The simplest explanation would be that of tidal interaction between galaxies, the gas being ripped out of the host galaxies in a close encounter. But how the six or seven supermassive concentrations without any stars can be formed in such an event still needs to be explained.”