In the early universe, galaxies formed out of clumps of matter, connected by filaments in a giant cosmic web. Within the galaxies, nuggets of gas cooled and condensed, becoming dense enough to trigger the birth of stars. Our Milky Way spiral galaxy and its billions of stars took shape in this way. The previous, standard model of galaxy formation held that hot gas sank into the centers of burgeoning galaxies from all directions. Gas clouds were thought to collide into each other, sending out shock waves, which then heated up the gas. The process is similar to jets creating sonic booms, only in the case of galaxies, the in-falling gas travels faster than the speed of sound, piling up into waves. Eventually, the gas cools and sinks to the galactic center. This process was theorized to be slow, taking up to 8 billion years.
Recent research has contradicted this scenario in smaller galaxies, showing that the gas is not heated. An alternate “cold-mode” theory of galaxy formation was proposed instead, suggesting the cold gas might funnel along filaments into galaxy centers. Kyle Stewart, Stewart, who is now at the California Baptist University in Riverside, Calif., completed the majority of this work while at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. and his colleagues set out to test this theory and address the mysteries about how the cold gas gets into galaxies, as well as the rate at which it spirals in.
"Galaxy formation is really chaotic," said Kyle Stewart, lead author of the new study appearing in the May 20th issue of the Astrophysical Journal. "It took us several hundred computer processors, over months of time, to simulate and learn more about how this process works."Since it would take billions of years to watch a galaxy grow, the team simulated the process using supercomputers at JPL; NASA’s Ames Research Center, Moffett Field, Calif.; and the University of California, Irvine.
Galaxy Evolution Chart - McDonald Observatory