An international team led by astronomers Gyula Józsa, Michelle Cluver, and Thomas Jarrett has utilized the South African MeerKAT telescope to discover a mysterious chain of hydrogen gas clouds the size of a massive galaxy. Indeed, the accumulation of so much elemental hydrogen without associated stellar components is the largest yet discovered. Appearing at the edge of a relatively massive group of galaxies, there is the possibility that the cloud chain is gas stripped from group-member galaxies, but it may also be primordial and gravitationally drawn into the group through a cosmic filament pathway. Whatever the case, MeerKAT is proving to be a ground-breaking telescope, and this “dark” cloud discovery should soon be followed by many such discoveries in the exciting days ahead.
The discovery of the mystery clouds will be published in the renowned Astrophysical Journal under the title: “The detection of a massive chain of dark HI clouds in the GAMA G23 Field”
Professor Cluver from Swinburne University explains: “Cosmic filaments are the highways along which mass concentrations come together under the action of gravity. 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. Seven spots of concentrated gas can be discerned from the complex. They form a huge complex of tenuous atomic hydrogen gas that stretches over a distance of 1.3 million light years. And indeed we found gas in many galaxies, but we did not expect anything like these clouds.”
The puzzling thing: Despite its extraordinarily large gas mass (equivalent to 10 billion of our Sun’s mass), there are close to no stars in the vicinity of the complex. Normally, any cool gas like atomic hydrogen is associated with their ‘home’ galaxy, where the gas reservoir continues to feed and grow the galaxy over eons of time. Such a large cloud simply cannot survive on its own: floating freely, the local environment is far too harsh as it gets heated up and ionised by the radiation from surrounding galaxies. Only the gravitational pull of a galaxy can, in principle, compact a cloud to a degree where it gets dense enough to create a natural shield against the cosmic background radiation. Professor Jarrett from the University of Cape Town: “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. There have to be stars, it’s inconceivable for a cloud the size of a galaxy to simply be floating in space!”
There is one possible exception to this, where a small dwarf galaxy is seen close to one corner quadrant of the cloud complex, leaving at least six dark clouds. Yet it is far far too small to possibly be the origin of all this gas. Professor Józsa from the Max-Planck-Institute for Radio Astronomy and Rhodes University explains: “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. The simplest explanation would be that of a tidal interaction between galaxies, the gas being ripped out of the host galaxies in a close encounter. But how the six of seven supermassive concentrations without any stars can be formed in such an event still needs to be explained.” Also, no obvious connection to any donor galaxy is evident from the data. The existence of these “dark clouds” is hence a mystery yet to be solved. Maybe the complex enters the group for the first time, consisting of primordial gas gravitationally pulled into the galaxy group along a cosmic filament. Now that the clouds have been discovered, the researchers hope that it will be possible to learn about their origin using dedicated, much deeper, observations with several telescopes in the southern hemisphere. Solving this puzzle could have interesting implications for our understanding of the role that atomic hydrogen plays in how galaxies evolve in large scale structures.
The most massive hydrogen-cloud — with a negligible amount of associated stars — discovered to date, found with the MeerKAT Habitat of Galaxies Survey (MeerHOGS), an initiative to exploit the sensitivity and 1-degree field of view of the MeerKAT SKA Precursor to map local large scale structures of the Cosmic Web. This 3D rendered video shows the cloud chain (at center) and its immediate surroundings. It starts with the spatial (X-Y) plane, rotating to view the velocity (Z-dimension) plane which shows the kinematic action of the gas. Typically a “cloud” will have a disk shape with rotation, but in the case of the “dark cloud”, there is little if any rotation, and is spread out along the spatial axis in a chain of dense concentrations. The central concentration is large enough to be its own galaxy, yet it contains no stars or activity of recent star formation. Read more about it in Józsa et al. 2021