Dark Matter Halos Are Sad Would-Be Galaxies

Physicists based out of Durham University have simulated the evolution of our entire local galactic group for the very first time. The results reveal how clumps of dark matter, called halos, sculpted early galaxies—or more pertinently, how so many of these halos failed to produce galaxies.

Cosmologists have long known that theoretically, there are millions of dark matter halos gallivanting around our galactic neighborhood. Indeed, the stuff famously accounts for about 85 percent of the universe's total mass. But clearly, the majority of these halos haven't produced galaxies, or the night sky would be much brighter.

“We know there can't be a galaxy in every halo,” said Carlos Frenk, director of Durham's Institute for Computational Cosmology, in a statement. “The question is: ‘Why not?’”

The answer, essentially, is a matter of pacing. Based on their simulations, Frenk and his team say that the faster halos formed, the more likely they would be to hold onto gas clouds and form galaxies. Those that developed at a slower rate were subjected to the searing heat produced by the universe's first batch of stars. In other words, when the stars blinked into existence, they sterilized the gases in slow-forming halos, preventing them from cooling into galaxies.

“This is actually very similar to how natural selection works in biology,” the project's lead researcher Till Sawala told me in an email. “Genetic mutations begin as a random process. However, evolution then selects those characteristics that are best suited (or 'fittest') for a particular environment.”

“Here, the radiation by the first stars plays the role of the environment, and only the halos which grew the quickest are selected to form galaxies,” he said. “The radiation by the first stars then provides a selection mechanism, so the halos that end up hosting small galaxies, which can be observed, are a very special subset, with a narrow range of formation scenarios.”

At the risk of anthropomorphizing dark matter halos, it's important to note that the early, rapidly forming galaxies didn't have any one specific edge over slower halos.

"There is no particular mechanism that makes some halos grow faster than others," Sawala told me. "Halos grow at different rates at different times during their evolution, in a slightly random fashion, that is ultimately governed by the seeds of structure formation left over after the Big Bang. Every halo has its own, unique formation history."

The Durham team presented their findings today, at the Royal Astronomical Society's National Astronomy Meeting and used a supercomputer called “the Cosmology Machine” to study how these diverse formation histories. The machine has over 5,000 times the computing power of a regular PC, and upwards of 10,000 times the memory. These extraordinary parameters allowed the Durham team to simulate our galactic neighborhood more precisely than ever.

“We have been the first to simulate the Local Group with its entire galaxy population,” Sawala told me. “Previous studies of the Local Group … have largely been limited to pure dark matter simulations, which is to say that they consider only gravity. On very large scales, gravity is by far the strongest force, so the approximation is justified.”