It took nearly a year of high-powered number crunching on various supercomputers, but researchers from UC Santa Cruz and the Institute for Theoretical Physics in Zurich have finally produced a computer simulation of a galaxy that looks much like our own. That may not sound so huge on its face, but it actually is the first high-resolution simulation of its kind that has turned out a galaxy similar to the Milky Way, and it has rescued the prevailing “cold dark matter” cosmological model of how our disc galaxy formed from a good deal of doubt.
That doubt arose from the fact that when previous, lower-resolution models were run based on that cosmological model, a huge central bulge emerged in the galaxy–a bulge that is absent from all but the center of the Milky Way (another way of saying that: there was more bulge and less disc, whereas the Milky way is more disc, less bulge). This had some physicists thinking that perhaps there was a flaw in the cosmological model itself, which seemed incapable of producing via simulation the flat, spiral-armed qualities consistent with observations of our galaxy.
But the problem wasn’t with the model, it turns out, but with the simulation of star formation. In reality, star formation happens in clusters, where dense clouds of gas feed the process of star birth in fairly tightly defined regions. But in low-resolution simulations(resolution in this sense means the ability to track individual particles), gas densities tended to spread out over relatively large areas, showing stars forming throughout the galaxy rather than in clusters. This led to a larger galactic bulge–and a less accurate picture of how Milky Way-like galaxies came into being.
To get the high resolution necessary to make the model work took a great deal of computing power, including 1.4 million processor-hours on NASA’s Pleiades supercomputer as well as additional time on supercomputers at UC Santa Barbara and the Swiss National Supercomputing Center. And at the time, the researchers had no idea if their added resolution would really make a difference.
It turns out it did. The simulated galaxy, Eris, shares the shape, bulge-to-disk ratio, star content, brightness, and various other characteristics with the Milky Way, demonstrating that the “cold dark matter” model can produce spiral-armed disc galaxies like the one we call home after all.