All around us in the universe there are billions of galaxies, but a key question remains unanswered about them: How exactly did they form?
Astronomers from the California Institute of Technology (Caltech) now think they may have a definitive answer, after finding a “smoking gun” of evidence for something known as the cold-flow model. This involves a cosmic web, composed of streams of dust and gas in the interstellar medium, funneling material to galaxies. This not only feeds them, but also drives their rotation.
Evidence for this theory comes from the new observation of a protogalaxy – the stage a galaxy goes through before it becomes a fully-fledged galaxy. Ten billion light-years away, a giant swirling disk of gas is being fed by a filament of the cosmic web, and a nearby quasar – a region of superheated dust and gas – called UM287 is illuminating the cosmic meal in action. The disk being fed is already about 400,000 light-years across, four times bigger than the Milky Way.
"This is the first smoking-gun evidence for how galaxies form," said Christopher Martin, professor of physics at Caltech and lead author of the new paper, in a statement. "Even as simulations and theoretical work have increasingly stressed the importance of cold flows, observational evidence of their role in galaxy formation has been lacking."
The team used the Cosmic Web Imager (CWI) at the Palomar Observatory in San Diego County, California, to make the discovery, to be published on August 13 in the journal Nature.
The cold-flow model is one of two that had been proposed for how galaxies form. The other was that halos of dark matter surrounding galaxies would gradually gather dust and gas from the interstellar medium, slowly building these huge structures. But the process was deemed too slow to account for the rate of star formation in the early universe.
The cold-flow model solves this problem by suggesting that the filaments deliver gas at a high velocity, forming a large rotating disk that leads to rapid star formation. And this latest discovery seems to show just that, with one side of the disk moving swiftly away from us and the other side towards us, with a rotation speed of about 400 kilometers (250 miles) per second.
"The filament has a more or less constant velocity. It is basically funneling gas into the disk at a fixed rate," said Matt Matuszewski, an instrument scientist at Caltech and co-author on the paper, in a statement. "Once the gas merges with the disk inside the dark-matter halo, it is pulled around by the rotating gas and dark matter in the halo."
More work will of course be needed to confirm that the observation and theory are correct. But, for now, it provides a compelling solution to how galaxies in the universe came to be.