Galaxy with 'missing' dark matter baffles scientists

In a first, scientists have discovered a galaxy that is almost completely devoid of dark matter - the mysterious substance believed to make up most of the universe. The galaxy, known as NGC1052-DF2, has been classified as an an ultra-diffuse galaxy, a relatively new type of galaxy that was first discovered in 2015. Ultra-diffuse galaxies are common, but no other galaxy of this type yet-discovered is so lacking in dark matter, researchers said.

"Finding a galaxy without dark matter is unexpected because this invisible, mysterious substance is the most dominant aspect of any galaxy," said Pieter van Dokkum of Yale University in the US. "For decades, we thought that galaxies start their lives as blobs of dark matter. After that everything else happens: gas falls into the dark matter halos, the gas turns into stars, they slowly build up, then you end up with galaxies like the Milky Way," said Van Dokkum, lead author of the study published in the journal Nature.

"NGC1052-DF2 challenges the standard ideas of how we think galaxies form," he said. The research used data from the Gemini North and W M Keck Observatories in Hawai'i, the Hubble Space Telescope, and other telescopes around the world. "NGC1052-DF2 is an oddity, even among this unusual class of galaxy," said Shany Danieli, graduate student at Yale University.

To peer even deeper into this unique galaxy, the team used the Gemini Multi Object Spectrograph (GMOS) to capture detailed images of NGC1052-DF2, assess its structure, and confirm that the galaxy had no signs of interactions with other galaxies. "Without the Gemini images dissecting the galaxy's morphology we would have lacked context for the rest of the data," said Danieli. "Also, Gemini's confirmation that NGC1052-DF2 is not currently interacting with another galaxy will help us answer questions about the conditions surrounding its birth," she said.

Researchers first spotted NGC1052-DF2 with the Dragonfly Telephoto Array, a custom-built telescope in New Mexico that they designed to find these ghostly galaxies. NGC1052-DF2 stood out in stark contrast when comparisons were made between images from the Dragonfly Telephoto Array and the Sloan Digital Sky Survey (SDSS).

The Dragonfly images show a faint "blob-like" object, while SDSS data reveal a collection of relatively bright point-like sources. To further assess this inconsistency the team dissected the light from several of the bright sources within NGC1052-DF2 using Keck's Deep Imaging Multi-Object Spectrograph (DEIMOS) and Low-Resolution Imaging Spectrometer (LRIS), identifying 10 globular clusters. 

These clusters are large compact groups of stars that orbit the galactic core. The spectral data obtained on the Keck telescopes showed that the globular clusters were moving much slower than expected.
The slower the objects in a system move, the less mass there is in that system. The team's calculations show that all of the mass in the galaxy could be attributed to the mass of the stars, which means there is almost no dark matter in NGC1052-DF2.

"If there is any dark matter at all, it's very little. The stars in the galaxy can account for all of the mass, and there doesn't seem to be any room for dark matter," Van Dokkum said. The team's results demonstrate that dark matter is separable from galaxies. "This discovery shows that dark matter is real - it has its own separate existence apart from other components of galaxies," said Van Dokkum.