In science, we often give provisional existence status to entities that serve as explanatory concepts to explain phenomena, even though they have not been directly detected as yet. Of course, that state of provisional existence does not last forever and strenuous efforts are undertaken to obtain direct measurements of existence and sometimes these can take a long time. That seems to be the case with dark matter which is thought to be about five times as abundant as normal matter and to exist in large spheres in which all galaxies are immersed but whose direct detection has proven elusive.
But now comes a report that astronomers have detected a galaxy that does not contain any dark matter at all. The paper was published in the March 29, 2018 issue of the journal Nature (Pieter van Dokkum et. al. A galaxy lacking dark matter, vol. 555, pages 629–632) that you can read here. This news report describes what they found.
Stupefied astronomers on Wednesday unveiled the first and only known galaxy without dark matter, the invisible and poorly-understood substance thought to make up a quarter of the Universe.
…“For a galaxy this size, it should have 30 times as much dark matter as regular matter,” [co-author Roberto Abraham, an astronomer at the University of Toronto] told AFP by phone. “What we found is that there is no dark matter at all.”
…Some 65 million light-years from Earth, NGC1052-DF2 — “DF2” for short — is about the same size as our Milky Way, but has 100 to 1,000 times fewer stars.
…The discovery was made with a new kind of telescope developed by Abraham and lead author Pieter van Dokkum of Yale University.
Unlike mirror-based devices, the mobile Dragonfly Telescope Array is composed entirely of nano-coated lenses, 48 in all.
“Conventional telescopes are good at finding small, faint objects. Ours is really good at finding large ones,” said Abraham.
Indeed, over the last few years Dokkum and Abraham have used it to uncover a whole new category of sparsely populated “ultra diffuse galaxies” — and sparked a cottage industry as astronomers struggle to explain their strange properties.
“Everything about them is a surprise, starting with the very fact they exist,” Abraham said.
Up to now, the analysis of galaxies has shown a fairly tight ratio of dark to ordinary matter. But this new class “is breaking all the rules,” he said.The first anomalies discovered were galaxies almost entirely composed of dark matter. That was odd enough.
But the real shocker was DF2, which has virtually none at all.
DF2 was first identified by Russian astronomers conducting a photographic survey, but it’s uniqueness didn’t come to light until later.
Dokkum’s team used the Keck telescopes in Hawaii to track the motion of several star clusters — each with about 100,000 stars — within the galaxy.
The clusters, they found, travelled at the same speed as the galaxy, itself moving through the Universe. Had there been dark matter, the clusters would be moving slower or faster.
For some background, it is currently believed that ‘normal’ matter and energy, i.e., those that we are familiar with such as atoms and molecules and the more elementary particles that comprise them such as protons, neutrons, electrons, and all the other particles whose existence has been established, constitute a surprisingly small percentage of all the mass and energy that make up the universe, just about 5% in fact. Dark matter is a new kind of matter whose existence has not yet been positively detected but only inferred and is thought to make up about 26% of the mass-energy of the universe. Dark energy is a new form of energy that is similarly postulated to exist and makes up the remaining 69% of the universe.
The idea of dark matter originated with the detection of anomalies in the velocities of stars in galaxies, with them having values that differed from predictions based on the observed amount of familiar matter that we knew about. The proposed solution was that there was a significant amount of matter in the universe that we had not detected and accounted for, and that it was this new matter that caused this anomalous behavior.
This idea had been around from the first quarter of the twentieth century but received a major boost in the 1970s with more careful measurements using new technology by Vera Rubin and co-workers of the speeds of stars in the spiral arms of galaxies. These more precise data could not be explained using existing, well-established theories of gravity and the ‘visible’ (i.e., known) amounts of matter but could be explained by postulating that galaxies were immersed in a sphere of matter that we had not detected as yet that extends well beyond the visible edge of the galaxies. The mass of this dark matter has been postulated as the source of the gravitational force that is needed to produce this anomalous behavior. Subsequent measures of stellar phenomena have bolstered the idea of such matter existing.
But finding direct confirmation has been elusive, mainly because we do not know what makes up dark matter and hence how it might interact with the ordinary matter which make up the detectors built to look for signs of its existence. The most sensitive experiments to date for the direct detection of dark matter, the LUX collaboration experiment in the US and the PandaX-II Collaboration in China have, within the last two years, said that they got null results.
So what does this new study imply? That no dark matter exists? If so, how do we explain the observations in every other galaxy? That dark matter does not exist only for this galaxy? Explaining a unique anomaly will be difficult. Or perhaps there are many galaxies like this one and that would require a different theory of dark matter formation and aggregation.
What is likely to happen now is that other astronomers will look at this same galaxy to see if the anomaly holds up while others try to find other galaxies with similar properties while theorists scramble to create new models that could explain either outcome.
Fun times!
This looks like a real problem for modified gravity theories (which don’t include dark matter).
Rob,
Yes, this new observation poses a problem for MOND (Modified Newtonian Dynamics) theories too because they were proposed to explain the same observations that dark matter sought to explain. In fact, this creates more difficulties for them since their’s is a general theory that would find it hard to accommodate this new finding.
Actually, the Bullet Cluster is an example of gas being separated from dark matter on an intergalactic scale. Two galaxy clusters collide. The stars and dark matter of the clusters pass through each other. But the gas within the clusters interacts, concentrating at the collision point. Perhaps new galaxies can form within this dark matter depleted gas.
… the ‘visible’ (i.e., known (i.e., calculated)) amounts of matter …
FTFY.
As for D2, the astronomers need ‘scopes capable of reading the signs forbidding presence of dark matter placed at each entry to that galaxy by the Kosmic Ku Klux Klan.
Come on, MS, you already know the correct answer: goddiddid. Evilutionism is falsified once again, science can’t answer it and changes all the time and this galaxy is obviously Intelligently Designed. If desired you can add that this galaxy proves that the Big Bang didn’t happen and that our Universe is 6000 years old.
@mnbo:
Goddidit is a fine explanation, since god is the universal answer: “We don’t know, therefore “. Since there is no evidence whatsoever that dark matter exists (all we know is that our gravitational equations don’t always work out), I’d be confused if the sacred yak didn’t have its hoof in there somewhere.
Whoops.. therefore *(insert your made-up deity of choice here)
I shouldn’t have used less than and greater than signs as delimiters.
The whole mass-energy content of the universe thing really ruins the old joke: Don’t trust atoms -- they make up everything.
Has anyone considered that the Xeelee might be winning the war on the Photino birds?
The whole dark matter/dark energy idea just feels wrong to me. Not that my feelings have any effect on the truth.
Mano, could you answer a question I’ve been having? Why does it seem that, locally, we are only affected by the gravitational pull of “normal” matter? The Earth revolves around the sun, as do the other planets. If there is something like 5x more dark matter permeating our solar system, why aren’t the planets whizzing around wildly due to the pull of dark matter’s gravity? Why am I accelerated toward the Earth at 9.81m/s^2, which is due to the pull of normal matter, without any influence by dark matter? Is there just no dark matter in our solar system? Is that unlikely?
@Marshall:
That one’s actually not all that difficult. Even if there is five times the amount of dark matter as there is normal matter within our solar system, it’s diffused out over the entire solar system out past the Oort Cloud, which is mostly empty space for normal matter. Not only does that mean that there’s significantly less effect from dark matter than there is from the solar wind and radiation pressure; but because it’s fairly evenly spread out near where we are, any real gravitational effects are pretty much cancelled out, just like there’s no net gravity if you’re at the centre of the Earth.
Think of dark matter particles as something akin to even more ghost-like neutrinos: zipping around at close to the speed of light and passing through everything (including themselves) on the way. They’d spread out into a thin broth all across the universe without gravity, because they don’t naturally bind together in atomic structures like normal matter. In order to get any real clumpiness in dark matter, you pretty much need gravity on the scale of a galactic core to hold it.
At least, that’s my understanding.
Marshall @#10,
To add to Lenora’s explanation above, the speed of the Earth’s orbit around the Sun is largely determined by the amount of mass in a sphere that is centered at the Sun and has a radius that is the distance from the Sun to the Earth. That is quite a small volume, cosmically speaking. The total mass of dark matter in a sphere that contains Saturn’s orbit is estimated to be less than 1.7x10--10 times the mass of the Sun, which is negligible.
But the Sun’s orbit around the center of our Milky Way galaxy is determined by the amount of matter in a sphere that is centered about the center of the galaxy and has a radius from that center to our galaxy. That volume (and hence mass) is much larger and thus has a larger effect but that affects the motion of the Solar system as a whole and does not much influence the relative motions within the system.
This is why the effect of dark matter is mostly seen in the outer arms of spiral galaxies, far from the galactic centers.
This also explains why the gravitational force on the surface of the Earth is determined by the Earth’s mass.