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Starts With A Bang

5 Ways To Make A Galaxy With No Dark Matter

NGC 1052-DF2 was said to ‘defy theory,’ incorrectly, by many. Here’s what our current theories actually predict.

Last week, astronomers announced the discovery of NGC 1052-DF2: a galaxy without dark matter.

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. 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 missing most, if not all, of its dark matter.(NASA, ESA, and P. van Dokkum (Yale University))

While most galaxies have much more dark matter than normal matter, this one, found nearby a giant elliptical, has practically none.

The giant elliptical galaxy NGC 1052 (at left) dominates the cluster it’s a part of, though many other large galaxies are present. Nearby, a small, barely-visible ultra diffuse galaxy, known as NGC 1052-DF2 (or just DF2 for short) appears to be made of normal matter alone.(Adam Block/Mount Lemmon SkyCenter/University of Arizona)

It’s the first of its kind, but doesn’t defy theories. Here are 5 ways to make “DF2.”

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. (NASA, ESA, Jean-Paul Kneib (Laboratoire d’Astrophysique de Marseille) et al.)

1.) From stripped gas in a cluster. When galaxies speed through the intra-cluster medium, their gas can get stripped out, creating new stars in isolation, without dark matter.

NGC 3561A and NGC 3561B have collided and produced huge stellar tails, plumes and even possibly “ejecta” that are condensing to make tiny “new” galaxies. Hot young stars glow blue where rejuvenated star formation is taking place. Part of a massive galaxy cluster, this ejected material could lead to the formation of a galaxy very similar to what we see for DF2. (Adam Block/Mount Lemmon SkyCenter/University of Arizona)

2.) Ejected from galactic mergers. When two galaxies smash together, they usually merge entirely, but sometimes there is ejected material. Sufficient amounts could create a baryons-only galaxy.

While distant host galaxies for quasars and active galactic nuclei can often be imaged in visible/infrared light, the jets themselves and the surrounding emission is best viewed in both the X-ray and the radio, as illustrated here for the galaxy Hercules A. A large outflow could eject a small galaxy’s worth of material, possibly leading to the creation of a dark-matter-free galaxy or collection of stars down the road. (NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA))

3.) Quasar outflows. The outflows from supermassive-black-hole-powered galaxies can recollapse to form their own galaxies. Normally, they create dwarf galaxies, but DF2 is potentially larger.

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 an objects could create a galaxy like DF2. (NASA, ESA, W. Keel (University of Alabama), and the Galaxy Zoo Team)

4.) Evolved Voorwerpjes. Galaxies sometimes have green, glowing companions: stripped and ionized material.

Astronomers found that Hanny’s Voorwerp is the only visible part of a 300,000-light-year-long gaseous streamer stretching around the galaxy. The greenish Voorwerp is visible because a searchlight beam of light from the galaxy’s core illuminated it. This beam came from a quasar, a bright, energetic object that is powered by a black hole. An encounter with another galaxy may have fed the black hole and pulled the gaseous streamer from IC 2497. (NASA, ESA, and A. Feild (STScI))

DF2 could be an evolved, 10,000,000,000 year old analogue of Hanny’s Voorwerp.

In theory, the majority of dark matter in the galaxy exists in a vast halo engulfing us, or, alternatively, the law of gravity is different on large scales. A galaxy without dark matter can have different properties from one with dark matter, but if dark matter isn’t real, why would two galaxies obey different laws of gravity? (ESO / L. Calçada)

5.) There is no dark matter. And, somehow, some galaxies might feel normal gravity from the normal matter, rather than obeying MOND.

The extended rotation curve of M33, the Triangulum galaxy. These rotation curves of spiral galaxies ushered in the modern astrophysics concept of dark matter to the general field. NGC 1052-DF2 very closely obeys the curve illustrated by the dashed line, showing a lack of dark matter. (Wikimedia Commons user Stefania.deluca)

If all galaxies follow the same underlying rules, only their compositions can differ.

The observed curves (black points) along with the total normal matter (blue curve) and various components of stars and gas that contribute to the rotation curves of galaxies. Both modified gravity and dark matter can explain these rotation curves, but a no-dark-matter galaxy obeying unmodified gravity rules now challenges MOND. (The Radial Acceleration Relation in Rotationally Supported Galaxies, Stacy McGaugh, Federico Lelli and Jim Schombert, 2016)

A dark-matter-free galaxy implies a dark-matter-rich Universe.

Mostly Mute Monday tells the astronomical story of an object, image, or phenomenon in pictures, visuals, and no more than 200 words.

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


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