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Starts With A Bang

Controversial ‘Dark Matter Free Galaxy’ Passes Its Most Difficult Test

Remember the announcement of a galaxy with no dark matter? It’s just been re-examined, with astounding results.

In theory, all galaxies should contain copious amounts of dark matter, with one exception.

One of the fastest known galaxies in the Universe, speeding through its cluster (and being stripped of its gas) at a few percent the speed of light: thousands of km/s. Trails of stars form in its wake, while the dark matter continues on with the original galaxy. Because normal matter responds to all of the Universe’s forces while dark matter only experiences gravitational forces, they can be separated from one another, with the stripped gas potentially recollapsing to form a smaller dark matter-free galaxy. (NASA, ESA, JEAN-PAUL KNEIB (LABORATOIRE D’ASTROPHYSIQUE DE MARSEILLE) ET AL.)

Galactic mergers, interactions, or gas stripping events can isolate large amounts of normal matter.

Star formation, gas bridges, and irregularly shaped galaxies are just some of the features arising in Hickson Compact Group 31. Compact groups can often illustrate how galaxy mergers appear in a variety of stages and circumstances, and could settle down into a large elliptical galaxy with a number of smaller satellite galaxies, some of which should contain dark matter but others of which should be devoid of dark matter entirely. (NASA / STSCI / WIKISKY / HUBBLE AND WIKIMEDIA COMMONS USER FRIENDLYSTAR)

These liberated clumps should gravitate and recollapse, creating dark matter-free galaxies.

Hanny’s Voorwerp, identified in 2011, was the first of some 20-odd objects now known to be a collection of green, glowing gas (because of ionized oxygen) that extends for tens of thousands of light years found outside of by nearby galaxies. An evolved version of such objects, which ought to represent a clump of normal matter ejected from a host galaxy, could create a dark matter-free galaxy like DF2 and DF4 are suspected to be. (NASA, ESA, W. KEEL (UNIVERSITY OF ALABAMA), AND THE GALAXY ZOO TEAM)

Detractors argued their absence proved dark matter’s non-existence.

The giant elliptical galaxy NGC 1052 (at left) dominates the cluster it’s a part of, though many other large galaxies are present, such as the giant spiral NGC 1042. Nearby to these galaxies are small, barely-visible ultra diffuse galaxies, known as NGC 1052-DF2 and NGC 1052-DF4 (or just DF2 and DF4 for short) which appear to be made of normal matter alone if they’re at the distance of NGC 1052: 60 to 70 million light-years away. (ADAM BLOCK/MOUNT LEMMON SKYCENTER/UNIVERSITY OF ARIZONA)

However, 2018 and 2019 saw scientists announce two dark matter-free galaxies: NGC 1052-DF2 and NGC 1052-DF4.

The full Dragonfly field, approximately 11 square degrees, centred on NGC 1052. The zoom-in shows the immediate surroundings of NGC 1052, with NGC 1052–DF2 highlighted in the inset. This is Extended Data Figure 1 from the van Dokkum et al. publication in 2018 announcing the discovery of DF2. A few months later, galaxy DF4 was measured in the same field, also revealing a dark matter-free nature. (P. VAN DOKKUM ET AL., NATURE VOLUME 555, PAGES 629–632 (29 MARCH 2018))

The key advance was measuring their velocity dispersions, an achievement enabled by modern instrumentation.

The Dragonfly telephoto array, which is optimized for finding extremely low-surface-brightness galaxies in the relatively nearby Universe (within 100 million light years), has found a unique galaxy in NGC 1052-DF2. It may be the first of multiple galaxies, now joined by NGC 1052-DF4, to show no evidence for dark matter inside of it. (ABRAHAM & VAN DOKKUM (2014) / YALE UNIVERSITY)

Those measurements, combined with parent galaxy NGC 1052’s distance, allowed reconstruction of their total masses.

The KCWI spectrum of the galaxy DF2 (in black), as taken directly from the new paper at arXiv:1901.03711, with the earlier results from a competing team using MUSE superimposed in red. You can clearly see that the MUSE data is lower resolution, smeared out, and artificially inflated compared to the KCWI data. The result is an artificially large velocity dispersion inferred by the prior researchers. (SHANY DANIELI (PRIVATE COMMUNICATION))

With distances estimated at ~60–70 million light-years, their galactic properties indicate a dark matter-free composition.

The galaxy NGC 1052-DF2 was imaged in great detail by the KCWI spectrograph instrument aboard the W.M. Keck telescope on Mauna Kea, enabling scientists to detect the motions of stars and globular clusters inside the galaxy to unprecedented precisions. (DANIELI ET AL. (2019), ARXIV:1901.03711)

Previously, a direct measurement of NGC 1052-DF2’s surface brightness fluctuations yielded that same distance, but doubts persisted.

This large, fuzzy-looking galaxy is so diffuse that astronomers call it a “see-through” galaxy because they can clearly see distant galaxies behind it. The ghostly object, catalogued as NGC 1052-DF2, doesn’t have a noticeable central region, or even spiral arms and a disk, typical features of a spiral galaxy. But it doesn’t look like an elliptical galaxy, either, as its velocity dispersion is all wrong. Even its globular clusters are oddballs: they are twice as large as typical stellar groupings seen in other galaxies. All of these oddities pale in comparison to the weirdest aspect of this galaxy: NGC 1052-DF2 is very controversial because of its apparent lack of dark matter. Joined by DF4, these galaxies could solve an enormous cosmic puzzle. (NASA, ESA, AND P. VAN DOKKUM (YALE UNIVERSITY))

A different team claimed these galaxies belong to a closer foreground group, implying a typical dark matter density.

The ultra-diffuse galaxy KKS2000]04 (NGC1052-DF2), towards the constellation of Cetus, was considered to be a galaxy completely devoid of dark matter. The results of Trujillo et al. dispute that, claiming that the galaxy is much closer, and therefore has a different mass-to-luminosity ratio (and a different velocity dispersion) than was previously thought. This is extremely controversial. (TRUJILLO ET AL. (2019))

Hubble measurements of the other galaxy, NGC 1052-DF4, were taken to settle the controversy.

The new data on galaxy NGC 1052-DF4, as taken with the Hubble Space Telescope by the team of Danieli, Van Dokkum and others, goes eight times deeper than previous observations. Measuring stars from the tip of the red giant branch, they infer a distance of 18.8 Mpc (61 million light-years) for the galaxy, consistent with the NGC 1052 group and not consistent with a separate foreground group some 20 million light-years closer. (S. DANIELI ET AL., SUBMITTED TO APJ LETTERS (2019))

Using stars from the tip of the red giant branch, a distance of 61 million light-years was determined, consistent with no dark matter.

Many nearby galaxies, including all the galaxies of the local group (mostly clustered at the extreme left), display a relationship between their mass and velocity dispersion that indicates the presence of dark matter. NGC 1052-DF2 is the first known galaxy that appears to be made of normal matter alone, and was later joined by DF4 earlier in 2019. These latest observations confirm the distance to these galaxies, and therefore confirm their dark matter-free nature. (DANIELI ET AL. (2019), ARXIV:1901.03711)

If these galaxies truly possess the sizes, distances, and velocity dispersions measured, they must be dark matter-free.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.