An intergalactic cloud has slammed into the Milky Way Galaxy and left a big hole.

Our galaxy is full of gas. It’s a good thing, too, because that’s what stars are made of. But this gas isn’t evenly distributed: sometimes it clumps into gigantic, dense clouds, other times it’s carved out by stellar outbursts or supernovae.

Among the largest cavities are supershells. These structures are big empty bubbles in a galaxy’s gas — so big, they can easily be 2,000 light-years wide. It takes a whole lot of energy to inflate that kind of bubble: the equivalent of at least 30 supernovae, planted like little vicious bombs at the cavity’s center.

But oddly, most of the supershells we know of in the Milky Way (about 20) or in nearby galaxies don’t have aging star clusters at their centers. So what blew them up, if not stars?

One alternative is high-velocity clouds. These clouds shower the Milky Way in a steady drizzle, helping to fuel star formation. We’re not sure if they’re debris from torn-up satellite galaxies, or stuff spewed out by our own supernovae, or gunk from the greater cosmos. Whatever they are, if such a cloud rammed into a galaxy, it could blast out a big bubble.

Geumsook Park (Seoul National University, South Korea) and colleagues have now found the first solid example of a high-velocity cloud doing just that. Astronomers already knew of the cloud, called HVC 040+01-282 (for its coordinates), which lies below the plane of the Milky Way’s disk in the outer galaxy, near the Scutum-Centaurus arm. This high-velocity cloud is of the “compact” kind, as opposed to being large and extended, so it’s nicknamed CHVC040 (the C is tacked on for “compact”).

As part of the I-GALFA survey with the Arecibo radio telescope, the team discovered a supershell surrounding CHVC040. The shell is a whopping 3,000 light-years wide, with neat spoke-looking structures inside it to boot. And smack-dab in its center sits CHVC040.

The team estimates in an upcoming Astrophysical Journal Letters paper that, if the cloud collided with its max possible velocity, it would have had more than enough energy to blow out the supershell.

Coauthor Bon-Chul Koo (Seoul National University) explains that supershells play an important role in galaxy ecology. They serve as "chimneys" that spew the heavier elements produced by supernovae in the disk into a galaxy’s halo, changing the gas’s composition. They can also heat and stir up the interstellar medium, or trigger star formation by compressing gas.

CHVC040 is the first high-velocity cloud linked to one of these bubble structures. But it’s probably not unique. Astronomers know of about 300 compact HVCs in the Milky Way, and many are stretched out into comet-like shapes, presumably as they burrow through the hot gas surrounding our galaxy in a big halo. There could be many more such pairings.

Reference:

G. Park et al. “A High-Velocity Cloud Impact Forming a Supershell in the Milky Way.” Accepted to Astrophysical Journal Letters. Prepublication draft available here.