Galaxy without dark matter discovered
Astronomers led by Pieter van Dokkum have discovered a galaxy that barely contains any dark matter, which actually proves that dark matter does indeed exist. The research results will be published this week in Nature.
Van Dokkum is Vice-Dean at the Faculty of Physics at Yale University in the United States. From 1997 to 1999, he worked at Leiden University as a PhD candidate. In 1999, he obtained his doctorate under Marijn Franx at the University of Groningen with a study on the formation and evolution of galaxies.
For their newly published research, Van Dokkum and his colleagues used a telescope with over 40 telephoto lenses; the biggest refracting telescope in the world, located in New Mexico State. He developed the Dragonfly Telephoto Array together with Canadian Professor Roberto Abraham.
Van Dokkum explains: ‘Regular telescopes are built to obtain sharp images, and significant sums are invested in them. Our relatively cheap telescope is made up of telephoto lenses for professional photographers. These lenses cannot produce such sharp images, but they hardly contain any background noise. As a result, we can discover new galaxies.’
Extremely faint galaxy
The newly discovered galaxy was named NGC 1052-DF2 and is at a distance of 60 million light years from Earth. It hardly contains any dark matter, unlike what astronomers would expect based on their models. The galaxy is the same size as our own. Up till now, it was overlooked because it contains 200 fewer stars than the Milky Way, and is thus extremely faint.
Alternative theories of gravity don’t work
Such galaxies were already known, but they all showed lots of dark matter. Van Dokkum: ‘The discovery is relevant because some scientists assume that dark matter doesn’t exist, and is caused instead because gravity works differently at large distances. To date, such theories could explain the observations surprisingly well. However, they do make the assumption that all galaxies must contain dark matter. This galaxy doesn’t, which makes it the first direct evidence that alternative gravitational theories cannot be applied to galaxies.’
The mysterious dark matter is manifested by galaxies that orbit around too quickly in relation to their mass; the amount of gravity is always more than what there should be based on visible matter. However, the new galaxy does not move fast. Van Dokkum: ‘All mass derives from stars and interstellar gas clouds within the galaxy. There doesn’t seem to be room for dark matter.’
The result also has consequences for the new theory of gravity by Erik Verlinde, a theoretical physicist from Amsterdam. After all, Verlinde’s theory predicts that all galaxies must contain ‘apparent dark matter.’
Image above article: Galaxy NGC 1052-DF2 without dark matter. This donkere materie. This is an image made by the Hubble Space Telescope. Copyright: STScI/Hubble
The research team with the Dragonfly telescope. Pieter van Dokkum is on the right. Copyright: Pieter van Dokkum