Rapidly moving gas is zipping around the Milky Way hundreds of times faster than the speed of sound in air. Known as high-velocity clouds (HVCs), these cold, hydrogen-packed, absolutely enormous entities aren’t just found in our own galaxy, but in several others.
A lot about them remains uncertain, but they’re important. The fact that they contain so much matter suggests that they’re a key part of the evolution of galaxies, so mapping them out would be a good shout. Fortunately, a researcher at the International Centre for Radio Astronomy Research (ICRAR) has done just that.
It seems that the darkness above our heads is busier than it may first appear. According to astronomer Dr Tobias Westmeier, who is also attributed to the University of Western Australia, about 13 percent of the visible sky is smothered by these HVCs.
This incredibly accurate map charting the locations of these HVCs was conducted as part of the HI4PI all-sky survey. Using two of the planet’s largest steerable radio telescopes found in Germany and Australia, both hemispheres were covered.
Millions of separate images and magnitudes more points of data – along with plenty of data processing to remove unrelated signals – revealed the locations of all the major HVCs we currently know of in unprecedented detail. Essentially, any vast clouds of hydrogen moving at a different speed from the Milky Way's spin were singled out.
Prior to this work, many of the HVC complexes were hard to resolve with any precision, and their shapes were a little ambiguous. Now, it appears that many are actually part of an “intricate network of narrow [hydrogen-rich] filaments and clumps.”
The map reveals that the clouds are within 30,000 light-years of our galaxy, which, in astronomical terms, is incredibly close. Still, this could mean that the clouds are being emitted from the Milky Way, or they’re falling into it.
“We can only measure the radial component of the velocity, i.e. how fast a cloud is moving towards us or away from us,” Westmeier told IFLScience.
In addition, because researchers use the positions of stars to indirectly place the locations of these HVCs, “the distances of most clouds are either unknown or only poorly constrained, so we have a rough idea where they are, but we don’t know exactly.”
Westmeier explained that, because of these problems, it’s “impossible to reconstruct a trajectory, which is part of the reason for why the nature of most of these clouds has remained a mystery for so many decades.”
This means we can't be sure where they started, or where they'll ultimately end up.
There is one exception to this. Some particularly ginormous HVCs are being jettisoned from the Large and Small Magellanic Clouds – satellite galaxies to our own – right through the Galactic South Pole of the Milky Way. The HVCs here are so expansive, clocking in at tens of thousands of light-years in length, that they’re collectively referred to as the Magellanic Stream.
Although they were discovered back in 1965, and linked back to the Clouds in 1974, they’ve only now been painstakingly mapped in relation to the other HVCs in the galaxy. In this case, it’s quite clear where they’re originating from, but their form and structure cannot be entirely explained at present.
In any case, HVCs are unlikely to be passing objects. They contain an awful lot of (mostly electrically neutral) hydrogen, which hypothetically means that they could be contributing to star formation if they travel to the right places.