In the dense, star-forming gas cloud W44, the ALMA telescope has spotted eight tiny clumps racing at enormous speeds. In position–velocity maps, each one looks like the letter V, just like the previously discovered 'Bullet'. This resemblance hints at a common origin.
Most likely, it wasn't a single object that dove into the cloud at high speed, but an entire small cluster of compact objects — possibly the remnants of an old star cluster. Their total mass, estimated from the velocity dispersion, is about one hundred thousand solar masses — roughly that of a typical globular cluster. Momentum analysis suggests the main 'Bullet' could have been created by a lone black hole that punched straight through the gas.
Such 'bullets' offer a rare glimpse of how invisible dense objects interact with the gas in the Galactic disk. They leave behind not only a hole but also a hot trail that radio telescopes can detect.
as written by the authors arXiv:2601.12728
We report the discovery of multiple compact molecular features exhibiting extremely broad velocity widths toward the W44 molecular cloud. ALMA CO $$J$$=3--2 data reveal eight ``Petit--Bullets'' surrounding the previously known ``Bullet.'' Each Petit--Bullet shows a distinct V-shaped structure in position--velocity space, reminiscent of the Y-shaped morphology of the Bullet, suggesting a common origin. These features are interpreted as the result of high-velocity plunges of compact gravitational objects into dense molecular gas. The spatial and kinematic properties of the Petit--Bullets suggest that the plunging material was not a single object but rather a small cluster of compact bodies. A virial mass of $$1.0\!\times\! 10^{5}\, M_\odot$$ inferred from their velocity dispersion is comparable to that of typical globular clusters. Momentum analysis further implies that the main Bullet likely formed by an isolated black hole. These findings provide new evidence for dynamical interactions between halo clusters and disk molecular gas.
The line widths of the 'bullets' correspond to speeds up to tens of kilometers per second — faster than a rifle bullet.