Astronomers have discovered a never-before-seen cluster of hydrogen clouds strewn between two nearby galaxies, Andromeda (M31) and Triangulum (M33).
The researchers speculate that these rarefied blobs of gas - each about as massive as a dwarf galaxy - condensed out of a vast and as-yet undetected reservoir of hot, ionised gas, which could have accompanied an otherwise invisible band of dark matter.
The astronomers detected these objects using the National Science Foundation's Green Bank Telescope (GBT) at the National Radio Astronomy Observatory (NRAO).
"We have known for some time that many seemingly empty stretches of the Universe contain vast but diffused patches of hot, ionised hydrogen," said Spencer Wolfe of West Virginia University in Morgantown.
"Earlier observations of the area between M31 and M33 suggested the presence of colder, neutral hydrogen, but we couldn't see any details to determine if it had a definitive structure or represented a new type of cosmic feature.
"Now, with high-resolution images from the GBT, we were able to detect discrete concentrations of neutral hydrogen emerging out of what was thought to be a mainly featureless field of gas," Wolfe said.
Astronomers are able to observe neutral atomic hydrogen, which is referred to as HI (H and the Roman numeral one), because of the characteristic signal it emits at radio wavelengths, which can be detected by radio telescopes on Earth.
Though this material is abundant throughout the cosmos, in the space between galaxies it can be very tenuous and the faint signal it emits can be extremely difficult to detect.
A little more than a decade ago, astronomers had the first speculative hints that a previously unrecognised reservoir of hydrogen lay between M31 and M33. The signal from this gas, however, was too faint to draw any firm conclusions about its nature, origin, or even certain existence.
Last year, preliminary data taken with the GBT confirmed that there was indeed hydrogen gas, and a lot of it, smeared out between the galaxies.
New and more thorough studies of this region with the GBT revealed that the hydrogen gas was not simply in the form of wispy streamers, as would be expected by the interactions of two galaxies in a gravitational ballet.
Instead, a full 50 per cent of the gas was conspicuously clumped together into very discrete and apparently self-gravitating blobs that - apart for their lack of stars - would be dead ringers for dwarf galaxies.
The GBT was also able to track the motion of these newly discovered clouds, showing that they were travelling through space at velocities similar to M31 and M33.
The study was published in the journal Nature.
