Dark matter is invisible, betraying its presence only through its gravitational pull. Without dark matter holding them together, our galaxy’s speedy stars would fly off in all directions. The nature of dark matter is a mystery; a mystery that a new study has only deepened.
“After completing this study, we know less about dark matter than we did before,” said lead author Matt Walker, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.
Dwarf galaxies are composed of up to 99 percent dark matter and only 1 percent normal matter like stars. This disparity makes dwarf galaxies ideal targets for astronomers seeking to understand dark matter.
Walker and his co-author Jorge Penarrubia (University of Cambridge, UK) analyzed the dark matter distribution in two Milky Way neighbors: the Fornax and Sculptor dwarf galaxies. These galaxies hold one million to 10 million stars, compared to about 400 billion in our galaxy. The team measured the locations, speeds and basic chemical compositions of 1500 to 2500 stars.
Their measurements showed that in both cases, the dark matter is distributed uniformly over a relatively large region, several hundred light-years across. This goes completely against the current cosmological models, which suggest that dark matter should be clustered at the centers of galaxies.
“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter ‘pit’ at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Penarrubia.
The study was published Monday in, The Astrophysical Journal and is available online.