Dark dwarfs

The standard cold dark matter model of cosmology is successful at explaining the large-scale structure of our universe. It nicely explains why we there are clusters and superclusters of galaxies, as seen in the recent image from the CFHTLenS collaboration.

However, the standard cold dark matter model makes a clear prediction that is not borne out by observation: it predicts that large galaxies should be attended by hundreds or even thousands of dwarf companions. They aren’t. Consider our Local Group of galaxies, for example. Andromeda and the Milky Way galaxy are the two stand-out members of the Group; Triangulum, the third largest galaxy, is tiny by comparison. Between them, they have about 30 to 50 companions. The same goes for other groupings: dwarf galaxies certainly exist, but in nowhere near the numbers predicted by the cold dark matter model.

So does that mean the standard cold dark matter model of cosmology is wrong?

Not necessarily. For instance, if those dwarf galaxies consisted primarily of dark matter then we wouldn’t see them, right?

A paper by Simona Vegetti and co-workers in today’s Nature, entitled Gravitational detection of a low-mass dark satellite galaxy at cosmological distance, describes an object that may be a dark dwarf galaxy. Vegetti and her co-workers studied the gravitational lensing caused by JVAS B1938+666 (a the giant elliptical galaxy at a redshift of 0.88). When viewed in the infrared, the galaxy generates a beautiful Einstein ring: the galaxy’s mass bends the light from an even more distant source and creates a distorted image.

An Einstein ring

The gravitational lensing system B1938+666 forms an Einstein ring in infrared light
Credit: D. Lagattuta and W.M.Keck Observatory

Vegetti and her colleagues have a detailed computer model of the gravitational lens. Their model suggests that an object with a mass 200 million times that of the Sun must be present in the system: but there’s no visible sign of such an object. One interpretation is that there’s a dwarf galaxy here whose main constituent is dark matter.

So perhaps, as the cold dark matter model predicts, dwarf galaxies are indeed formed in large numbers. It’s just that most of them are made of the most abundant form of matter in the universe: dark matter. We simply don’t see them.