Instead, astronomers suspect that some of them were created by high-velocity clouds
Also known as Smith's Cloud, it's one of thousands of high-velocity clouds
of hydrogen gas flying around the outskirts of our Galaxy.
The Andromeda finding strengthens the case that the high-velocity clouds
surrounding the Milky Way include remnant gas from the galaxy's origin, says Thilker.
"The Smith Cloud is unique among high-velocity clouds
because it is so clearly interacting with and merging with the Milky Way," said Felix J.
The halo clouds are distinct from a larger population of 'high-velocity clouds
' that also sail outside the galaxy.
The fact is, many other "high-velocity clouds" are also falling toward the galactic plane with no one noticing except specialists.
Accurate distances are known for only a handful of high-velocity clouds. With the distance known, it was straightforward to calculate the cloud's dimensions--11,000 light-years long by 2,500 wide--and its position with respect to the Milky Way: some 9,000 light-years south of the galactic plane and 25,000 from the galactic center.
Complex H's radio emissions show that the object is much closer than high-velocity clouds and that its motion is tied to that of the Milky Way.
Astronomers had assumed the body was a so-called high-velocity cloud, a type of fast-moving mass of atomic hydrogen commonly found far from the galaxy.
Judging from models of structure formation in which small objects in the cosmos coalesce to form larger ones, "there ought to be clouds of dark matter and gas falling into our galaxy, and that's what we believe these high-velocity clouds
are," he says.
The origin of these so-called high-velocity clouds
had been uncertain since they were first detected by Dutch astronomer Jan Oort in 1961.
Astronomers believe that without this protective shell, the high-velocity cloud
(HVC) known as the Smith Cloud would have disintegrated long ago when it first collided with the disk of our Galaxy.