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

The hazy blob on the heart 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 yr later, the identical crew of astronomers discovered its twin, a equally darkish matter-much less galaxy dubbed NGC 1052-DF4.NASA / ESA / P. van Dokkum (Yale University)Two years in the past, a deep picture of the area round a shiny soccer-form galaxy referred to as NGC 1052 turned up one thing utterly completely different: a fluffy dwarf galaxy that apparently lacked darkish matter. Then, a yr later, the identical crew of astronomers introduced one other darkish matter-much less galaxy in the identical area.

The two, which go by NGC 1052-DF2 and -DF4, respectively, turned the thought of galaxy evolution on its head. Dark matter should be the galactic mould, creating the gravitational effectively that bizarre matter falls into to make galaxies. But if a galaxy is lacking darkish matter, how can it kind within the first place?

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

Going Deep

Using the Gran Telescopio Canarias and the IAC80 telescopes in Tenerife, Spain, in addition to archival photographs from the Hubble Space Telescope, Montes’ crew took a deep have a look at little DF4. These photographs reveal a faint S­­-shape to the galaxy’s outer limbs.

A deep picture of the darkish matter-much less dwarf NGC 1052-DF4 reveals its slight S-form. The bigger spiral galaxy to the left is NGC 1035. (Note that this picture is a composite of the colour picture and the inverted shade picture; the inverted shade helps spotlight faint, diffuse mild. The S-form seen on this inverted-shade area of the picture 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 quite a relief to see these kinds of distortions,” says Pieter van Dokkum (Yale), who was not concerned on this examine however led the crew that found DF2 and DF4. The unique photographs his crew 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 crew is now taking extremely-deep observations of their very own. “We find qualitatively similar results,” he says.

However, 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 the complete 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 could clarify DF4’s lack of darkish matter. Like most molds, darkish matter extends past the type of the galaxy. The Milky Way, for instance, spans some 100,000 mild-years, however its darkish matter halo would possibly prolong 10 occasions farther out. So when two galaxies go 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-form tails that 7% of the galaxy’s stars have flowed out into the tails already. That implies that the darkish matter has largely leaked away whereas the bigger galaxy’s pull is just starting to have an effect on the celebrities.


Other tidally disrupted dwarf galaxies have been noticed in recent times that additionally lack darkish matter, though these had been extra clearly galaxies in transition. The issue of seeing DF4’s tidal tails have made the reply on this case extra elusive. This, Montes and her crew argue, makes the examine of faint particulars important to understanding such distant, dim programs. (Montes’ crew didn’t embrace observations of the opposite galaxy, DF2, on this examine.)

A Mystery Remains

However, whereas the galactic thief state of affairs explains the dearth of darkish matter, it would not handle why these galaxies (DF4 and DF2 alike) have such lamp-like globular clusters, historical balls of densely packed stars that orbit most galaxies.

A survey picture taken with the Dragonfly Telephoto Array reveals objects throughout the discipline of the elliptical galaxy NGC 1052 (heart), together with DF2 (backside left) and DF4 (high 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 University) / STScI / ACSVan Dokkum’s crew initially estimated the overall mass of those puny galaxies by their globular clusters’ orbits, after which in contrast that mass to the sunshine they might see from the galaxies’ stars. They concluded that the galaxies had been darkish matter-much 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 proven fact that the globulars had been so shiny. Each one is as luminous as Omega Centauri, the very brightest globular cluster orbiting the Milky Way, and altogether they make up an unusually giant share of the galaxies’ mass. (Montes and her crew dispute this, as they measure a better distance to each 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. But the globular clusters — if they’re the truth is distinctive — recommend in any other case. “Something must have happened to form these galaxies,” van Dokkum says, “because tidal distortions don’t explain the globular clusters.”

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

Further Reading:

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

Editorial observe (Dec. 8, 2020): This publish has been up to date with extra data from Pieter van Dokkum (Yale) and Mireia Montes (University of New South Wales, Australia).

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