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A Dark Matter-less Galaxy, Explained (Maybe) – Sky & Telescope

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The first dark matter-less galaxy, NGC 1052-DF2
The hazy blob on the middle of this Hubble picture is the weird galaxy NGC 1052–DF2, a small fluffy galaxy lacking most — if not all — of its darkish matter. A 12 months later, the identical staff of astronomers discovered its twin, a equally darkish matter-less galaxy dubbed NGC 1052-DF4.
NASA / ESA / P. van Dokkum (Yale College)

Two years in the past, a deep picture of the area round a vibrant football-shape galaxy often known as NGC 1052 turned up one thing fully totally different: a fluffy dwarf galaxy that apparently lacked dark matter. Then, a 12 months later, the identical staff of astronomers introduced another dark matter-less galaxy in the identical area.

The 2, which go by NGC 1052-DF2 and -DF4, respectively, turned the concept of galaxy evolution on its head. Darkish matter should be the galactic mildew, creating the gravitational nicely that odd matter falls into to make galaxies. But when a galaxy is lacking darkish matter, how can it type within the first place?

The reply, Mireia Montes (College of New South Wales, Australia) and colleagues write within the December 1st Astrophysical Journal, is that the galaxies weren’t at all times lacking darkish matter. Trying extra carefully at DF4, they conclude that the gravitational pull of one other close by galaxy — spiral-shape NGC 1035 — stripped it away.  

Going Deep

Utilizing the Gran Telescopio Canarias and the IAC80 telescopes in Tenerife, Spain, in addition to archival pictures from the Hubble House Telescope, Montes’ staff took a deep take a look at little DF4. These pictures reveal a faint S­­-shape to the galaxy’s outer limbs.

Close-up on the dark matter-less dwarf
A deep picture of the darkish matter-less dwarf NGC 1052-DF4 reveals its slight S-shape. The bigger spiral galaxy to the left is NGC 1035. (Be aware that this picture is a composite of the colour picture and the inverted colour picture; the inverted colour helps spotlight faint, diffuse gentle. The S-shape within the inverted-color area of the picture, seen as a slight extension of the black shading to the highest proper and the underside left of the galaxy, comes from stars flowing away from the galaxy as a result of tidal pull of neighboring NGC 1035.
M. Montes et al. / Astrophysical Journal 2020

“It was fairly a reduction to see these sorts of distortions,” says Pieter van Dokkum (Yale), who was not concerned on this research however led the staff that found DF2 and DF4. The unique pictures his staff had taken hadn’t picked up any distortions and that had fearful him, as a result of with out darkish matter the galaxy should be flying aside. His staff is now taking ultra-deep observations of their very own. “We discover qualitatively related outcomes,” he says.

Nonetheless, regardless of agreeing on the information, van Dokkum disagrees with the interpretation that one other galaxy is pulling on DF4. The galaxy ought be ripped aside regardless whether or not it has a galactic neighbor, he explains, as a result of it exists not in isolation however inside a galaxy group. The mixed gravitational fields of your entire group creates an atmosphere such that a feeble galaxy like DF4 (or DF2, for that matter, or any galaxy with out the stabilizing impact of darkish matter) ought to not survive.

If NGC 1035 is doing the pulling, it might clarify DF4’s lack of darkish matter. Like most molds, darkish matter extends past the type of the galaxy. The Milky Method, for instance, spans some 100,000 light-years, however its darkish matter halo would possibly lengthen 10 occasions farther out. So when two galaxies cross one another by, the bigger one pulls on the smaller galaxy’s darkish matter halo first, earlier than it pulls any stars.

Montes and colleagues estimate from the extraordinarily faint glow of the S-shape tails that 7% of the galaxy’s stars have flowed out into the tails already. That signifies that the darkish matter has largely leaked away whereas the bigger galaxy’s pull is just starting to have an effect on the celebrities.


Different tidally disrupted dwarf galaxies have been noticed lately that additionally lack darkish matter, though these had been extra clearly galaxies in transition. The problem of seeing DF4’s tidal tails have made the reply on this case extra elusive. This, Montes and her staff argue, makes the research of faint particulars important to understanding such distant, dim methods. (Montes’ staff didn’t embody observations of the opposite galaxy, DF2, on this research.)

A Thriller Stays

Nonetheless, whereas the galactic thief situation explains the dearth of darkish matter, it does not deal with why these galaxies (DF4 and DF2 alike) have such lamp-like globular clusters, historic balls of densely packed stars that orbit most galaxies.

DF2 and DF4
A survey picture taken with the Dragonfly Telephoto Array exhibits objects throughout the discipline of the elliptical galaxy NGC 1052 (middle), together with DF2 (backside left) and DF4 (prime proper). DF2 and DF4 are related in lots of properties, together with measurement, luminosity, morphology, and their luminous globular cluster populations.
P. van Dokkum (Yale College) / STScI / ACS

Van Dokkum’s staff initially estimated the entire mass of those puny galaxies by their globular clusters’ orbits, after which in contrast that mass to the sunshine they may see from the galaxies’ stars. They concluded that the galaxies had been darkish matter-less as a result of the mass from each strategies was the identical — all of the mass was within the stars.

The astounding conclusion of no darkish matter obscured the opposite oddity: The truth that the globulars had been so vibrant. Every one is as luminous as Omega Centauri, the very brightest globular cluster orbiting the Milky Method, and altogether they make up an unusually massive proportion of the galaxies’ mass. (Montes and her staff dispute this, as they measure a closer distance to both DF2 and DF4, which they are saying explains the globular clusters’ odd properties.)

If a galaxy stole DF4’s darkish matter, that may implies its evolution proceeded as regular till it bumped into the galactic thief. However the globular clusters — if they’re actually distinctive — counsel in any other case. “One thing will need to have occurred to type these galaxies,” van Dokkum says, “as a result of tidal distortions do not clarify the globular clusters.”

The talk over these little darkish matter-less galaxies is much from over.

Additional Studying:

A Galaxy Without (Much) Dark Matter
Globular Clusters Shine in a Galaxy Lacking Dark Matter
Why Are Two Ghost Galaxies Missing Dark Matter?

Editorial be aware (Dec. eight, 2020): This publish has been up to date with further data from Pieter van Dokkum (Yale) and Mireia Montes (College of New South Wales, Australia).

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