Using CSIRO’s Australian Square Kilometre Array Pathfinder (ASKAP) and background radio galaxies, astronomers have spotted a long cloud of cold hydrogen about 13 light-years away.
“We suspect that much of the ‘missing’ baryonic matter is in the form of cold gas clouds either in galaxies or between galaxies,” said co-lead author Yuanming Wang, a doctoral candidate in the School of Physics at the University of Sydney.
“This gas is undetectable using conventional methods, as it emits no visible light of its own and is just too cold for detection via radio astronomy.”
What Wang and colleagues did is look for radio sources in the distant background to see how they ‘shimmered.’
“We found five twinkling radio sources on a giant line in the sky,” Wang said.
“Our analysis shows their light must have passed through the same cold clump of gas.”
Just as visible light is distorted as it passes through our atmosphere to give stars their twinkle, when radio waves pass through matter, it also affects their brightness. It was this ‘scintillation’ that the astronomers detected.
“We aren’t quite sure what the strange cloud is, but one possibility is that it could be a hydrogen ‘snow cloud’ disrupted by a nearby star to form a long, thin clump of gas,” said co-lead author Dr. Artem Tuntsov, a researcher at Manly Astrophysics.
Hydrogen freezes at about minus 260 degrees Celsius (minus 436 degrees Fahrenheit) and theorists have proposed that some of the Universe’s missing baryonic matter could be locked up in these hydrogen clouds.
“However, we have now developed a method to identify such clumps of ‘invisible’ cold gas using background galaxies as pins,” Wang explained.
The hydrogen cloud detected by the team is about 0.3 light-years long, and has a mass about that of the Moon.
“It is ASKAP’s wide field of view, seeing tens of thousands of galaxies in a single observation that allowed us to measure the shape of the gas cloud,” said co-author Dr. Keith Bannister, a researcher at CSIRO Astronomy and Space Science.
“This is the first time that multiple ‘scintillators’ have been detected behind the same cloud of cold gas,” said co-lead author Professor Tara Murphy, a researcher in the Sydney Institute for Astrophysics at the University of Sydney and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).
“In the next few years, we should be able to use similar methods with ASKAP to detect a large number of such gas structures in our Milky Way Galaxy.”
The team’s paper was published in the Monthly Notices of the Royal Astronomical Society.
Yuanming Wang et al. ASKAP observations of multiple rapid scintillators reveal a degrees-long plasma filament. MNRAS, published online January 21, 2021; doi: 10.1093/mnras/stab139