Last updated March 28, 2018 at 1:36 pm
A newly discovered galaxy has astronomers puzzled with a complete absence of any dark matter. But it’s not the end of the dark matter story, just a new chapter.
The standard model of cosmology assumes that dark matter provides the gravitational backbone around which the galaxies form. Without this assumption, our understanding of the universe and its structure starts falling apart. But astronomers at the University of Toronto’s Dunlap Institute have discovered something never seen before – a dark matter-less dwarf galaxy seemingly formed entirely from atoms.
While the discovery challenges this dark matter assumption, it paradoxically also strengthens the case for dark matter.
A galaxy never seen before
A long known collection of luminous point-like objects, at a distance of 63 million light years from the Milky Way, termed NGC1052-DF2 attracted astronomers attention when the new Dragonfly telescope array, specifically designed to identify ultra-faint light signals, revealed a different structure to what had been seen before.
Surrounding these points of light was in fact an extended smudge of light, too faint to have been noticed before.
The points of light are globular clusters, tightly bound collections of millions of stars. The smudge of light around them was an ultra-diffuse dwarf galaxy, with 200 million times the mass of our Sun.
Such a marked difference is enough to pique any astronomer’s attention. The most powerful optical telescopes on the ground and space, the Keck and Hubble Space Telescope respectively, were turned to this unusual but until this moment mostly unremarkable object. What they observed shook the foundations of modern galaxy formation.
The gravity from the dwarf galaxy holds the globular clusters in their orbit, and by measuring the velocity of those globular clusters we can infer the amount of mass within their orbits that must be present to provide that gravity. In galaxies like our Milky Way there is approximately twice as much mass required than we can see in all the stars and gas.
The inference is then that there exists a new type of other otherwise invisible material known as dark matter that makes up the difference, famously shown by American astronomer Vera Rubin nearly fifty years ago and the first indication of this hidden component.
For the smallest dwarf galaxies the difference between the mass required and what we can see is far larger. Some objects are essentially just a few stars orbiting inside a vast halo of dark matter, outweighing the atoms 400 times over.
When researchers took a closer look at the globular clusters in NGC1052-DF2 they were astounded to see them barely moving at all, by galactic standards, buzzing around at less than 10.5 kilometres per second. This implies there is less than 340 million times the mass of our Sun within a sphere of radius 23,000 light years.
That amount of mass is almost the same as what we can account for in all of the stars that make up the newly revealed faint smudge of light. Indeed, within the errors there is in fact no dark matter present at all.
An absence of dark matter
This is astounding, the dark matter is assumed to be the gravitational backbone around and within which the galaxies grow.
So critical is it that some galaxy simulations on supercomputers only model the dark matter and add the atoms in later. The irony is however that far from ruling out the dark matter paradigm this observation in fact supports it.
As dark matter is a separate component it should be possible to have structures with only dark matter and not atoms (as has been seen) but also, in theory, atoms with no dark matter (as we have now uncovered).
Other theories that try to replace dark matter by modifying the physics of gravity itself, such as MOND, can’t explain a system that doesn’t seem to need the extra gravity.
Unlike dark matter which is a separate component, the modified gravity effect always has to be ‘on’ and therefore can’t explain this galaxy which doesn’t appear to need the additional force.
The obvious question is how does an ultra-diffuse galaxy entirely devoid of dark matter like NGC1052-DF2 actually form? It could be that atoms were blasted out of a nearby galaxy by the radiation from a feeding black hole, known as a quasar. Or infalling gas flows to the larger nearby galaxies instead fragmented into their own smaller galaxy rather than fall directly into the neighbour.
Either way it is likely that the unusually large and bright globular clusters in this system are related to its unusual formation scenario.
Fifty years ago a galaxy entirely composed of atoms was the expectation, now it’s the exception that proves the rule that we are living in a dark matter world.
The study has been published in Nature.