An international team of researchers using the NASA/ESA Hubble Space Telescope and several other observatories have, for the first time, uncovered a galaxy minus its dark matter. This discovery of the galaxy NGC 1052-DF2, detailed in the journal Nature, raises questions on the currently-accepted theories of and galaxy formation and provides new insights into the nature of dark matter.
The discovery of DF2 was made with a telescope developed by University of Toronto's astronomer Roberto Abraham and Pieter van Dokkum of Yale University. “This invisible, mysterious substance is by far the most dominant aspect of any galaxy. Finding a galaxy without any is completely unexpected; it challenges standard ideas of how galaxies work,” said lead author Pieter van Dokkum of Yale University.
“Dark matter is conventionally believed to be an integral part of all galaxies — the glue that holds them together and the underlying scaffolding upon which they are built,” explained study co-author Allison Merritt from Yale University in the US. “There is no theory that predicts these types of galaxies — how you actually go about forming one of these things is completely unknown.”
Van Dokkum's team tracked the motions of 10 bright star clusters and found that they were travelling way below their expected velocities and looked like they were almost standing still. Based on these data the team discovered that the newly discovered galaxy is larger than the Milky Way, but contains about 250 times fewer stars, leading it to be classified as an ultra diffuse galaxy.
“I spent an hour just staring at this image,” van Dokkum said as he recalled first seeing the Hubble image of NGC 1052-DF2. “This thing is astonishing: a gigantic blob so sparse that you see the galaxies behind it. It is literally a see-through galaxy,” van Dokkum noted. Further measurements of the dynamical properties of 10 globular clusters orbiting the galaxy allowed the team to infer an independent value of the galaxies mass.
This discovery is unpredicted by current theories on the distribution of dark matter and its influence on galaxy formation. The discovery of NGC 1052-DF2 nonetheless demonstrates that dark matter is somehow separable from galaxies. This is only expected if dark matter is bound to ordinary matter through nothing but gravity.
Van Dokkum and his team are now hunting for other dark matter-deficient galaxies, with three potential leads set to be explored in the coming months. But nevertheless, NGC1052-DF2 has baffled scientists. "Every galaxy we knew about before has dark matter, and they all fall in familiar categories like spiral or elliptical galaxies," van Dokkum noted. "But what would you get if there were no dark matter at all? Maybe this is what you would get."