The Weirdness of Dark-Matter Free Galaxies

Ultra Diffuse Galaxy


Welcome to our very strange Universe. And, yes, that is a galaxy. The discovery of yet another dark-matter free, ultra-diffuse galaxy, raises a number of unanswered questions for astronomers: how are they formed? What do they tell us about standard cosmological models? How common are they, and what other unique properties do they have? It will take the discovery of more dark-matter-less galaxies to resolve the ultimate question of what dark matter really is.

“The interesting thing is: we have no idea!”  says Pieter van Dokkum, Sol Goldman Professor of Astronomy at Yale University, who wrote in an email to The Daily Galaxy about why an ultra-diffuse galaxy contains no dark matter. “The existence of this galaxy shows that there is another pathway to creating galaxies than our standard picture, but what that might be is anyone’s guess.”

Evidence for a dark matter-free galaxy becomes stronger

Following van Dokkum’s finding about ultra diffuse galaxy DF2, an international team of astronomers led by researchers from the Netherlands has announced that they have found no trace of dark matter in a galaxy about the size of our own Milky Way, AGC 11405, despite taking detailed measurements over a course of forty hours with state-of-the-art telescopes. 

“We Have No Idea of Why They Exist” –Weird Dark-Matter-Free Galaxies 

When Pavel Mancera Piña (University of Groningen and ASTRON, the Netherlands) and his colleagues discovered six galaxies with little to no dark matter, they were told “measure again, you’ll see that there will be dark matter around your galaxy” reports Royal Astronomical Society (RAS). However, after forty hours of detailed observations using the Very Large Array (VLA) in New Mexico (United States), the evidence for a dark matter-free galaxy only became stronger. The techniques and measurements of Piña and colleagues are more robust according to the RAS.

The galaxy in question, AGC 114905, is about 250 million light-years away. It is classified as an ultra-diffuse dwarf galaxy, with the name ‘dwarf galaxy’ referring to its luminosity and not to its size. The galaxy contains a thousand times fewer stars than our Milky Way. The prevailing idea is that all galaxies, and certainly ultra-diffuse dwarf galaxies, can only exist if they are held together by dark matter –an essential ingredient of galaxies, but this finding obviously suggests that its presence may not be inevitable.

The researchers collected data on the rotation of gas in AGC 114905 for 40 hours between July and October 2020 using the VLA telescope. Subsequently, they made a graph showing the distance of the gas from the center of the galaxy on the x-axis and the rotation speed of the gas on the y-axis. This is a standard way to reveal the presence of dark matter. The graph shows that the motions of the gas in AGC 114905 can be explained by just normal, baryonic matter.

Is Dark Matter Only the Tip of an Invisible Universe of Unknown Forces?

“This is, of course, what we thought and hoped for because it confirms our previous measurements,” says Piña. “But now the problem remains that the theory predicts that there must be dark matter in AGC 114905, but our observations say there isn’t. In fact, the difference between theory and observation is only getting bigger.”

In their scientific publication, the researchers list the possible explanations for the lack of dark matter one by one. For example, AGC 114905 could have been stripped of dark matter by large nearby galaxies. Mancera Piña: “But there are none. And in the most reputed galaxy formation framework, the so-called cold dark-matter model, we would have to introduce extreme parameter values that are far beyond the usual range. Also with modified Newtonian dynamics, an alternative theory to cold dark matter, we cannot reproduce the motions of the gas within the galaxy.”

According to the researchers, there is one more assumption that could change their conclusions. That is the estimated angle at which they think they are observing the galaxy. “But that angle has to deviate very much from our estimate before there is room for dark matter again,” says co-author Tom Oosterloo (ASTRON).

Meanwhile, the researchers are examining a second ultra-diffuse dwarf galaxy in detail. If they again observe no trace of dark matter in that galaxy, it will make the case for dark-matter-poor galaxies even stronger.

The Last Word

In an email to The Daily Galaxy, Pavel Piña wrote:  “We do not know whether AGC 114905 has a peculiar population of globular clusters. We plan to obtain the observations that will allow us to explore that very soon, but so far we have no information whatsoever on the globular clusters of our galaxy, in contrast to van Dokkum et al who suggest that the ultra diffuse galaxies seem to have super luminous globular clusters, but this is something we do not know for sure about our ultra-diffuse AGC 114905. Astronomers like Shany Danieli have looked into this and found that the super-luminous globular clusters probably tell us that in the past UDGs had very compact star-forming regions. In general, those types of observations can be very helpful to understand the processes of star formation in UDGs.”

“On a different note,” Piña notes, “I think our new observations on AGC 114905 are very exciting because it looks like they can give us clues on the nature of dark matter itself, and thus have huge implications for our understanding of galaxy formation and evolution (and of the Universe in general).”

Astrophysicist Niayesh Afshordi at the Cosmology and Gravitation group at the Perimeter Institute for Theoretical Physic, wrote In an email to The Daily Galaxy, : “Discovery of ultra diffuse galaxies with apparent dearth of dark matter is an interesting twist in the story of galaxy formation. In my view, what this suggests is that we are yet to understand the diverse modes of galaxy formation that may occur at different epochs and environments. In particular, globular clusters that appear to be old spectators of galaxy formation today, might have played a more significant role in their youth.” 

Source: No need for dark matter: resolved kinematics of the ultra-diffuse galaxy AGC 114905 arXiv:2112.00017 [astro-ph.GA] Accepted for publication in Monthly Notices of the Royal Astronomical Society.

Image credit: The galaxy AGC 114905. The stellar emission of the galaxy is shown in blue. The green clouds show the neutral hydrogen gas. The galaxy does not appear to contain any dark matter, even after 40 hours of detailed measurements with state-of-the-art telescopes. Credit: (c) Javier Román & Pavel Mancera Piña

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Niayesh Afshordi, Pavel Mancera Piña, and the Royal Astronomical Society

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