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.