A Galaxy Without Dark Matter

A Yale-led research team has discovered a galaxy that contains no dark matter — a finding that confirms the possibility of dark matter as a separate material elsewhere in the universe.

The discovery has broad implications for astrophysics, the researchers said. It shows for the first time that dark matter is not always associated with traditional matter on a galactic scale, ruling out several current theories that dark matter is not a substance but merely a manifestation of the laws of gravity on cosmic scales.

“We thought that every galaxy had dark matter and that dark matter is how a galaxy begins,” said Pieter van Dokkum, Yale’s Sol Goldman Family Professor of Astronomy and lead author of a new study in the journal Nature.

“This invisible, mysterious substance is the most dominant aspect of any galaxy,” van Dokkum said. “So finding a galaxy without it is unexpected. It challenges the standard ideas of how we think galaxies work, and it shows that dark matter is real. It has its own separate existence apart from other components of galaxies. This result also suggests that there may be more than one way to form a galaxy.”

Researchers from Yale, San Jose State University, the University of Toronto, Harvard, and the Max Planck Institute for Astronomy collaborated on the study.

Van Dokkum’s team located the galaxy — NGC 1052-DF2 — with the Dragonfly Telephoto Array, a telescope van Dokkum invented and built with co-author Roberto Abraham of the University of Toronto. The galaxy had been cataloged previously, but the researchers noticed it looked quite different in Dragonfly images.

“It looked like a diffuse blob sprinkled with very compact star clusters,” said co-author Shany Danieli, a Yale graduate student. “I love working with the Dragonfly telescope, as it shows us faint structures that no one has even seen before.”

The researchers used the W. M. Keck Observatory in Hawaii to measure the motions of 10 very dense groupings of stars called globular clusters. They found that the clusters were moving at relatively low speeds — less than 23,000 miles per hour. Stars in galaxies containing dark matter move at least three times faster.

Using the new motion measurements, the researchers calculated NGC 1052-DF2’s mass. “If there is any dark matter at all, it’s very little,” van Dokkum explained. “The stars in the galaxy can account for all the mass, and there doesn’t seem to be any room for dark matter.”

The researchers used NASA’s Hubble Space Telescope and the Gemini Observatory in Hawaii to uncover more details about the galaxy. Gemini revealed that the galaxy does not show signs of an interaction with another galaxy; Hubble helped to identify the globular clusters and measure an accurate distance to the galaxy.

“I spent an hour just staring at the Hubble image,” van Dokkum said. “It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and say, ‘I’ve never seen that before.’ This thing is astonishing, a gigantic blob that you can look through. It’s so sparse that you see all of the galaxies behind it. It is literally a see-through galaxy.”

Reference: “A Galaxy Lacking Dark Matter,” Pieter van Dokkum et al., 2018 March 29, Nature [https://www.nature.com/articles/nature25767]. Additional co-authors of the study are Yotam Cohen and Lamiya Mowla of Yale; former Yale graduate student Allison Merritt, now at the Max Planck Institute for Astronomy; Aaron Romanowski of San Jose State University; Jean Brodie of the University of California-Santa Cruz; Charlie Conroy and Ewan O’Sullivan of Harvard; and Deborah Lokhorst of the University of Toronto.