Their model used supercomputers at the Naval Research Laboratory and the National Center for Supercomputing Applications at the University of Illinois to model large regions of space in great detail so that the picture resolves even very low-mass regions where stars might be inclined to form.
In an article to be published in the October 1 issue of Astrophysical Journal Letters, Kolatt and his colleagues argue that their high-power models provide evidence that young galaxies are formed in brief but energetic bursts of star-forming activity.
The argument, called the Collisional Starburst Scenario, opposes a popular explanation that galaxies accrete by slow, steady star formation over time. That explanation is known as the Central Quiescent theory of galaxy formation.
According to the best-accepted explanation of the universe -- the Cold Dark Matter Theory -- the cosmos consists primarily of invisible dark matter, which comprises almost 90 percent of the universe's mass. This dark matter travels through the universe in large blobs called 'dark matter halos'. Existing in their centers, galaxies are essentially trace particles for these halos -- bright sparks that mark the center of huge invisible blobs.
The Central Quiescent Theory holds that inside these halos, galaxies form slowly and incrementally. There, a large amount of gas is present and, gradually, it is turned into stars that, in turn, build galaxies. According to this model, galaxies should begin as rather dim objects and slowly brighten as their masses increase, that is, as more and more stars are formed.
The problem with this explanation is that it doesn't seem to account for some observations, in which apparently low-mass galaxies are extremely bright early in the history of the universe.
This creates a bit of a problem for the Central Quiescent theory. It doesn't explain how young, very low-mass galaxies could burn with the same intensity as a massive structure like the Milky Way.
This seeming discrepancy provided the niche where the Collisional Starburst Scenario developed.
It holds that not all galaxies are formed by gradual accretion of matter at the center of dark-matter halos. Many -- perhaps most -- are formed quickly by collisions between small clumps of matter that are not necessarily at the center of these halos.
Violent collisions between two objects could trigger a huge burst of star formation in which gas is converted into stars very efficiently, explains James Bullock, one of the proponents of the Collisional Starburst Scenario, and a co-author of the Astrophysical Journal Letters paper.
"Rather than having a very large galaxy that creates a lot of light because there's a lot of mass of gas there to be converted into stars, what you have is a very small amount of mass, but almost all of it is going into stars in a short amount of time," Bullock said.
In the latest computer scenario, when the Santa Cruz team sought to test whether models of the early universe would produce enough collisions to support Collisional Starburst, it did, said Bullock. Moreover, the number of starburst regions appears to be so great in that model of the early universe, that it appears to show that galaxies were not forming in the way predicted by the Central Quiescent theory, he said.
But other cosmologists seem unconvinced that the model can produce any clear answers about galaxy formation.
"At best, these are sort of theoretical cookbooks for making star formation, but we don't really have very much confidence that they describe reality," said Charles Steidel, an astronomer at California Institute of Technology who is one of the developers of the Central Quiescent Theory of galaxy formation. "It's just another model right now. I don't think that they're convincing a whole lot of people that this is the right model at this point."
One of the problems with any theoretical model of galaxy formation is that there is not enough good observational data to determine whether or not any conclusion is correct, Steidel warns, so in most models the result is more or less predetermined by the assumptions initially put in.
"All it means that they've put in a prescription that more or less guarantees that they need to get out this particular answer."
Other scientists working on structure formation in the universe express similar doubts. David Weinberg, a cosmologist in the astronomy department at Ohio State University, said his conclusions working with models of galaxy formation come down somewhere between the Central Quiescent and the Collisional Starburst models, but tend to be in closer agreement with the former.
Weinberg does credit the Santa Cruz group with providing an alternative interpretation that accounts for low-mass objects that are very bright though.
"What I find particularly interesting with the Santa Cruz model is that they show that there is this very different picture of what is going on that can, at least, account for some facts. At the moment it certainly seems like a viable one," he said.
"The biggest uncertainty in the theoretical modeling is our poor understanding of how star formation on a galactic scale actually works. Does it go in a steady, stately way, or is it an intermittent, very jumpy phenomenon?" said Weinberg.
"I think with future observations, probably we'll be able to distinguish which of those pictures is closer to correct, and what we will actually learn from that is how star formation really goes on a galactic scale," he observed.
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