“Smoking-gun evidence” of galaxies forming from a cold flow

Galaxies are pretty well understood on the whole. We know they’re made mostly of dark matter, we know how they move and rotate, and have grouped them into several classes. We even know a bit about the supermassive black holes that reside at their center. But one aspect of galaxies has remained vague: how they formed in the first place.

This is an important question, not only for the obvious reasons—we live in a galaxy, and we tend to be interested in our own origins—but also because the processes that formed galaxies play into the larger scale structure of the Universe. In the “cosmic web,” each fiber is made of many galaxies, and understanding the fine details of what’s going on within those fibers can help us better understand the web’s behavior as a whole.

One hypothesis about the formation of galaxies on this web is known as the “cold flow” model. It holds that galaxies form from flows of matter that are relatively cold, at about 10,000 kelvins. This diffuse material would stream along the web’s dark matter filaments, some of it ultimately getting stuck at the intersections among them. There, the matter would be attracted to dark matter halos and stream into them, forming into spinning disks of gas and dust—baby galaxies.