Every galaxy that we see in the Universe is thought to be embedded in a vast halo of "dark matter" - invisible matter that we can't see with our telescopes. In fact, the stars, dust and gas that we see in our own galaxy, the Milky Way, only account to about 10-20% of the Galaxy's total mass! The true shape of the distribution of such dark matter halos - whether they are more like giant dark matter soccer balls or more like "triaxial" American footballs, remains a mystery. A team of astronomers (led by graduate student Sarah Pearson) at Columbia have recently proposed a new approach to distinguish between these shapes.
Their method involves tracing the shape of stellar streams across the sky. Stellar streams form when stars leak out of star clusters, while they orbit as satellites around the Milky Way. Since the first observational discovery of a stellar stream in 1971 (the Arcturus stream), dozens of stellar streams have been discovered in our galaxy. Most of these faint streams are out in the halo of the Galaxy - far beyond most of the stars that make up the Milky Way. Such streams can get thousands of light years long, and some even wrap around the entire Galaxy. The shape and curvature of such streams give us insights on the gravitational field of the Galaxy.
In their recent publication, the authors investigated the thin stellar stream emerging from the star cluster Palomar 5, which orbits high above the stellar disk of our Galaxy. They found that the thin stream could not be reproduced in numerical simulations if the cluster was modeled in a galaxy where the dark matter distribution was assumed to be American football-shaped. This is unexpected since cosmological simulations of large-scale structure formation favor such American football-shaped halos for galaxies like the Milky Way.
In the case where the dark matter was distributed in a soccer-ball shaped halo the modeled stream of Palomar 5 traced the thin, curved shape of the stream we observe on the sky (see right Figure). However, the stream showed a peculiar morphology if the star cluster was modeled in a triaxial American football-shaped dark matter halo: it appeared fanned (see left Figure).
The authors conclude that this stream-fanning effect can give us important new insights on the dynamics of stars in galaxies like the Milky Way. They further conclude that the existence and location of thin streams like Palomar 5 can provide broad but powerful constraints on the shape of the dark matter distribution surrounding our Galaxy.