Dark Matter Is a No Show in a Ghostly Galaxy Far, Far Away

Galaxies and dark matter go hand in hand; you typically don’t find one without the other. (Credit: Gemini Observatory / NSF / AURA / Keck / Jen Miller)
Galaxies and dark matter go hand in hand; you typically don’t find one without the other. (Credit: Gemini Observatory / NSF / AURA / Keck / Jen Miller)

Astronomers, using data from the Gemini and W. M. Keck Observatories in Hawai’i, have encountered a galaxy that appears to have almost no dark matter. Since the Universe is dominated by dark matter and it is the foundation upon which galaxies are built, “…this is a game changer,” according to Principal Investigator Pieter van Dokkum of Yale University.

Galaxies and dark matter go hand in hand; you typically don’t find one without the other. So when researchers uncovered a galaxy, known as NGC1052-DF2, that is almost completely devoid of the stuff, they were shocked.

The research, published in the March 29th issue of the journal Nature, amassed data from the Gemini North telescope and W. M. Keck Observatory, both on Maunakea, Hawai’i, the Hubble Space Telescope, and other telescopes around the world.

Composite color image of NGC1052-DF2 constructed from observations using the Gemini Multi Object Spectrograph (GMOS) on Gemini North on Hawai‘i’s Maunakea. The ultra-diffuse galaxy was observed using deep imaging in two filters (g' and i'). Image credit: Gemini Observatory/NSF/AURA/Keck/Jen Miller

Composite color image of NGC1052-DF2 constructed from observations using the Gemini Multi Object Spectrograph (GMOS) on Gemini North on Hawai’i’s Maunakea. The ultra-diffuse galaxy was observed using deep imaging in two filters (g’ and i’). (Imaget: Gemini Observatory / NSF / AURA / Keck / Jen Miller)

Given its large size and faint appearance, astronomers classify NGC1052-DF2 as an ultra-diffuse galaxy, a relatively new type of galaxy that was first discovered in 2015. Ultra-diffuse galaxies are surprisingly common. However, no other galaxy of this type yet-discovered is so lacking in dark matter.

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.

Van Dokkum and his team 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.

In addition to the Gemini observations, 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 revealed 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.

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.

Left: The ultra-diffuse galaxy is rich in globular clusters, which hold the key to understanding this mysterious object’s origin and mass. Right: A closer look at one of the globular clusters within the galaxy, which are all much brighter than typical, the brightest emitting almost as much light as the brightest within the Milky Way. The spectrum, obtained by Keck Observatory shows the absorption lines used to determine the velocity of this object. Ten clusters were observed, providing the information needed to determine the mass of the galaxy, revealing its lack of dark matter. Image credit: Gemini Observatory/NSF/AURA/Keck/Jen Miller/Joy Pollard

Left: The ultra-diffuse galaxy is rich in globular clusters, which hold the key to understanding this mysterious object’s origin and mass. Right: A closer look at one of the globular clusters within the galaxy, which are all much brighter than typical, the brightest emitting almost as much light as the brightest within the Milky Way. The spectrum, obtained by Keck Observatory, shows the absorption lines used to determine the velocity of this object. Ten clusters were observed, providing the information needed to determine the mass of the galaxy, revealing its lack of dark matter. (Image: Gemini Observatory / NSF / AURA / Keck/ Jen Miller / Joy Pollard)

NGC1052-DF2’s globular clusters and atypical structure has perplexed astronomers aiming to determine the conditions this galaxy formed under.

However, researchers do have some ideas. NGC1052-DF2 resides about 65 million light years away in a collection of galaxies that is dominated by the giant elliptical galaxy NGC 1052. Galaxy formation is turbulent and violent, and van Dokkum suggests that the growth of the fledgling massive galaxy billions of years ago perhaps played a role in NGC1052-DF2’s dark-matter deficiency.

Another idea is that a cataclysmic event within the oddball galaxy, such as the birth of myriad massive stars, swept out all the gas and dark matter, halting star formation.

These possibilities are speculative, however, and don’t explain all of the characteristics of the observed galaxy, the researchers add.

The team continues the hunt for more dark-matter-deficient galaxies. They are analyzing Hubble images of 23 other diffuse galaxies. Three of them appear to share similarities with NGC1052-DF2, which van Dokkum plans to follow up on in the coming months at Keck Observatory.

Provided by: Gemini Observatory [Note: Materials may be edited for content and length.]

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