Tweak to dark matter may explain Milky Way mystery

Dozens of tiny galaxies known as satellite galaxies orbit the Milky Way, but theorists predict there should be hundreds.  Now a team of astronomers offers a new resolution to this conflict between observation and theory: Maybe dark matter, the mysterious substance thought to bind galaxies together, isn’t quite so dark. The researchers propose that radiation might have stirred up dark matter in the early universe, preventing the formation of satellites.

The mystery of the missing satellite galaxies has bedeviled astronomers for more than a decade. While dark matter has proven to be exceptionally good at explaining the formation of large galaxies and clusters, it routinely runs into trouble when attempting to describe tiny structures such as satellite galaxies.

Physicist Celine Bœhm of Durham University in England and colleagues sought to reconcile this discrepancy by tweaking dark matter theory. They propose that dark matter, long thought to be oblivious to light, actually interacts with photons in subtle ways. This interplay might have been important shortly after the Big Bang, when a smaller universe meant a much higher density of light, but it would be undetectable now because photons are spread thin after billions of years of cosmic expansion.

Bœhm’s team ran computer simulations to see how interactions between dark matter and light might affect the formation of satellites around a galaxy like the Milky Way. Their results, appearing September 8 in the Monthly Notices of the Royal Astronomical Society Letters, indicate that dark matter that is weakly sensitive to light could reduce the number of predicted satellite galaxies. In the case of the Milky Way, that reduction would bring the number much closer to what astronomers have observed.

No one has explored this particular idea before, says Erik Tollerud, an astrophysicist at Yale University unconnected to the research team.

But, he adds, there are a few caveats. For example, the simulations by Bœhm’s team predict many more fast-moving satellites than are seen.

What’s more, Tollerud says, many other ideas have been put forth to fix the missing satellite problem.  It’s possible that supernovas, the explosive deaths of massive stars, may stir up the gas in nascent galaxies, preventing them from forming stars. Also, tiny galaxies may be torn apart as they fall toward the Milky Way.

“With quasi-reasonable explanations, you can get about the right number of satellites,” Tollerud says, without mucking around with otherwise successful theories about dark matter.

Carlton Baugh, a Durham University cosmologist and coauthor of the new study, acknowledges that other forces may be at work. The new simulations, he says, are just meant to illustrate that photons, which are typically ignored when talking about dark matter, could play a role in suppressing the formation of satellites in the early universe.

One of the next goals for the team, Baugh says, is to add supernovas to the calculations. Figuring out the effects of supernovas might help the researchers fine-tune the possible relationship between dark matter and light and perhaps point to ways to finally capture an elusive dark matter particle.