Researchers using several observatories, including NASA’s Hubble Space Telescope, have for the first time uncovered a galaxy that is missing most, if not all, of its dark matter.
The finding flies in the face of traditional scientific belief, which claimed the invisible dark matter makes up the bulk of our universe and is the glue that holds visible matter together in galaxies.
“We thought that every galaxy had dark matter and that dark matter is how a galaxy begins,” said Pieter van Dokkum of Yale University in New Haven, Connecticut, lead researcher of the Hubble observations.
“This invisible, mysterious substance is the most dominant aspect of any galaxy. 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,” van Dokkum said.
The unusual galaxy, called NGC 1052-DF2, contains at most 1/400 the amount of dark matter that astronomers had expected. It is as large as our Milky Way, but it had escaped attention because it contains only 1/200 the number of stars. Given the object’s large size and faint appearance, astronomers classify it as an ultra-diffuse galaxy, a relatively new type of galaxy that was first discovered in 2015.
However, none of the ultra-diffuse galaxies discovered so far have been found to be lacking in dark matter. So even among this unusual class of galaxy, NGC 1052-DF2 is unique.
Van Dokkum and his team uncovered the dark-matter deficiency by using the WM Keck Observatory in Hawaii to measure the motions of 10 giant groupings of stars called globular clusters. They found that the globular clusters were moving at relatively low speeds, less than 23,000 miles per hour. Stars in normal galaxies containing dark matter move at least three times faster.
From those measurements, the team calculated the galaxy’s mass, concluding that “If there is any dark matter at all, it’s very little. The stars in the galaxy can account for all the mass, and there doesn’t seem to be any room for dark matter,” van Dokkum said.
The researchers also used Hubble and the Gemini Observatory in Hawaii to uncover more details about the unique object. Gemini revealed that the galaxy does not show signs of an interaction with another galaxy. Hubble helped them better identify the globular clusters and measure an accurate distance to the galaxy.
The Hubble images also revealed the galaxy’s unusual appearance. “I spent an hour just staring at the Hubble image,” van Dokkum recalled. “It’s so rare, particularly these days after so many years of Hubble, that you get an image of something and you 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.”
“There is no theory that predicted these types of galaxies. The galaxy is a complete mystery, as everything about it is strange. How you actually go about forming one of these things is completely unknown,” he added
“As someone who studies galaxy formation and evolution, this result suggests that there may be more than one way to form a galaxy. The discovery of Ultra Diffuse Galaxies (UDGs) in the Coma cluster in 2015 made us wonder how a galaxy can form with so much dark matter while this recent discovery of NGC 1052-DF2 begs the question of how a galaxy can form with so little dark matter. Neither of them can be formed with the current picture of galaxy formation and that’s what makes it more exciting,” said Lamiya Mowla, a Bangladeshi astrophysicist who is part of the discovery team at Yale University, USA.