In November, astronomers at the Chinese Academy of Sciences in Beijng published a paper identifying 19 galaxies which might violate the most fundamental theory of how the universe first formed.
They had been searching the sky for yet-undiscovered galaxies which seem to be lacking the usual dark matter component, aiming to add more evidence to a baffling phenomenon scientists had begun observing last year. And they claimed to have found a whole group of them.
Until recently, it was almost unanimously assumed that huge amounts of invisible dark matter was the key to galaxy formation; the gravitational effects experienced and induced by clumps of dark matter in the universe produced swirling disks of gas clouds, stars and dark matter. Dark matter had to make up the majority of matter in these galaxies, according to standard models of the universe, otherwise they would never have formed.
But since 2016, researchers keep stumbling upon galaxies that don’t seem to be dominated by dark matter. Some calculations imply that these galaxies lack dark matter entirely. Either way, galaxies which have far less dark matter than expected would give physicists a lot of explaining to do: why are they lacking dark matter, and how did they form in the first place?
Galaxies without dark matter could rewrite the history of the universe
Unlike the researchers in Beijing, Yale professor Pieter van Dokkum wasn’t trying to find anomalies among the "ultra diffuse" galaxies he studies—those with very few stars. After all, before van Dokkum made a chance discovery in 2018, there was very little reason to expect to find any galaxies which had formed and continued to exist without dark matter.
In fact, van Dokkum and his team expected to find dark matter everywhere they looked. Unlike our own star-packed Milky Way, where dark matter is only on the periphery, ultra-diffuse galaxies should be densely filled with dark matter. Their analysis instead identified, for the first time, a galaxy without any dark matter at all.
“The way the stars move should be entirely dominated by the amount of dark matter,” he says. “The fact we found this deficit was not a subtle result.”
Though van Dokkum's findings were and remain controversial, new analysis of galaxies observed through giant telescopes keeps poking holes in accepted theories. If nothing else, the latest study by researchers in Beijing highlights still-open questions about the shape and structure of galaxies.
Attempts to explain the apparent lack of dark matter in the identified dwarf galaxies fall into two broad categories: theories about how and why galaxies may have been stripped of the dark matter they once had, and theories which postulate the formation of galaxies without dark matter at all.
In the first group, researchers such as Go Ogiya at Observatoire de la Côte d'Azur in France are exploring a phenomenon called "tidal stripping" in which matter is pulled away from a galaxy due to tidal forces that arise when it is interacting with the gravitational field of another galaxy. Even before van Dokkum reported his observations of two dark-matter-deficient galaxies, there was evidence that tidal stripping would disproportionately affect dark matter, which is less tightly bound than visible or “baryonic” matter (i.e. stars) in the centre of the galaxy.
“Whether the baryonic matter is stripped from the satellite galaxy depends on its structure: the distribution of stars determines their resilience to the tidal force,” Ogiya wrote in an email. “In my numerical simulations…stars are hardly stripped from the satellite galaxy while its size doubles as a result of dark matter stripping.”
Ogiya also suggests a process by which galaxies might form without ever containing dark matter. So-called “tidal dwarf galaxies” could form when dark matter and baryonic matter is “ejected” from an existing galaxy due to tidal forces, but the dark matter component evaporates due to its higher velocity, leaving only stars and gas to form a new galaxy.
However, the theories with game-changing potential are those which rewrite the history of the early universe, around 13 billion years ago, long before the “heyday of galaxy formation” when tidal stripping might have taken place. Though he says he's agnostic about proposed explanations of dark-matter-deficiency, van Dokkum favours a theory suggesting that dark matter and gas interacted just after the Big Bang in ways which are not currently predicted. This could cause gas clumps which would separate from the dark matter and then later form galaxies made up only of gas and stars.
“That would be really cool because then we would learn something about the conditions of the early universe and probe the epoch when galaxy formation was not yet underway,” he says.
Dark matter deficiency, or miscalculation?
Some scientists believe that van Dokkum's result, while striking, may not be reliable. Since dark matter can’t be seen, how much of it is present in a galaxy has to be inferred from the behaviour of stars in the galaxy, which can be seen. Calculating this behaviour (for instance, velocity of the stars), depends on other galactic traits, like the shape of the galaxy and its distance away from us.
Ignacio Trujillo, a scientist specializing in Extragalactic Astronomy at the Institute of Astrophysics of the Canary Islands, has shown that the strange behaviour of these ultra-diffuse galaxies can be explained away by measuring how far away they are using alternative methods. If the anomalous galaxies are in fact much closer to Earth than the distance used for van Dokkum’s calculations, then calculating their dark matter components gives results within the expected range.
"You find yourself in this strange regime where you have to ask yourself: how strange a hypothesis can I accommodate which is still less strange than finding no dark matter?"
According to Trujillo, it is indeed possible for very tiny galaxies to form without the presence of dark matter, but not the way galaxies were formed just after the Big Bang. Rather than gas and stars clumping together due to gravitational effects, these galaxies form when an existing galaxy collides with another and residual matter stripped from the colliding galaxies forms a new galaxy—all without any dark matter. In contrast to galaxies formed by the tidal-stripping effects Ogiya studies, these galaxies would live for a relatively short time by cosmic standards: a few hundred years. Examples have already been identified for decades, he says, but these aren’t the sort of dark-matter-deficient galaxies that challenge the current understanding of our universe.
“People have already found things they have called galaxies without dark matter, but they are transient features which then disappear,” he says. “The novelty would be to find a galaxy which was formed originally in the early universe without dark matter, and I don’t think we have any strong candidates yet.”
Already, Trujillo’s colleagues say they have also found issues with the research published last month identifying 19 possible galaxies lacking dark matter. This week the American Astronomical Society published a research note authored by Jorge Sanchez Almeida which argued that the dark matter calculations for the 19 galaxies failed to account for true shape of dwarf galaxies, simplifying them as being “disk-like” instead of elliptical 3D shapes.
For van Dokkum, questions on both sides of the debate are very much still open. The two anomalous galaxies he’s identified as being dark-matter-deficient are strange in a number of ways, by anyone’s standards, he says. If they do in fact contain dark matter, contrary to his own hypothesis, other assumptions about the distribution of dark matter, as well as the shape and orientation of the galaxies and their history will need to be revised.
“You can come up with unlikely scenarios in which these galaxies do contain dark matter,” he says. “And unlikely is relative in this case to the hypothesis that there is no dark matter.”
“You find yourself in this strange regime where you have to ask yourself: how strange a hypothesis can I accommodate which is still less strange than finding no dark matter?”
The “fun thing” about the ultra-diffuse dwarf galaxies van Dokkum and others are studying is that they are “nearby and bright," van Dokkum said, which makes it relatively easy to investigate their behaviour.
With the powerful telescopes available today, van Dokkum hopes not only to make more accurate measurements of galaxy velocities and distance, but also identify the epoch in which the galaxies formed, using new data from the Hubble telescope. This empirical evidence would rule out a whole class of explanations for the apparent dark matter deficiency, one way or another.
Nicolas Martin, a researcher at the Observatory Astronomical De Strasbourg in France, believes that the observations needed to drive research forward are just beyond the limits of what is possible with the best apparatus around at the moment. Emailing while on location at one such cutting-edge telescope, he said that the research community would likely have to wait for two next-generation telescopes, currently planned or under construction in Chile and Hawaii, before they could generate even more precise measurements of the velocity of stars in the dwarf galaxies.
In the meantime, Martin, van Dokkum, Ogiya, and others will all be searching the sky to determine how many other strange dark-matter-deficient galaxies might be out there.
“If they are very common, then they need to be naturally produced by our models of galaxy formation,” Martin says. “If they are not common, then maybe they are just weirdos, coming from environments that are poorly understood corner cases, which is less problematic overall.”