Dark Matter is one of the tenacious mysteries facing astronomers and cosmologists today. This theoretical mass was proposed in the 1960s as a way to explain the rotational curves of galaxies, which indicated that they had greater mass than their stellar populations implied. Despite decades of research and observation, scientists have yet to find any direct evidence of this mysterious, invisible mass or what it is composed of. There are many theories, ranging from weakly interacting massive particles (WIMPs) to extremely low mass particles (axions).
Fortunately, we live in an era when the frontiers of astronomy are constantly being pushed and new discoveries are being made all the time. In a recent study, an international team of researchers led by the Leibniz Institute for Astrophysics Potsdam (AIP) has shed light on this decades-old debate by analyzing stellar velocities from 12 of the smallest and faintest galaxies in the Universe. The team found that the internal gravitational fields of these galaxies could not be explained by visible matter alone, further bolstering the case for Dark Matter.
The team was led by researchers from the AIP, and included members from the Institute for Physics and Astronomy at Potsdam University, University of Surrey, the University of Bath, the School of Astronomy and Space Science at Nanjing University, the Institute of Astrophysics and Space Sciences at the University of Porto, the Leiden Observatory at Leiden University, and the Lund Observatory at Lund University. The paper describing their findings recently appeared in the journal Astronomy & Astrophysics.
For decades, scientists have debated the existence of Dark Matter (DM). On the one hand, its existence is inferred from observations and our understanding of gravity (as described by Einstein’s Theory of General Relativity). On the other hand, there is a lack of direct evidence, which has led to alternative theories, such as Modified Newtonian Dynamics (MOND). This theory emerged in the 1980s and posits that the laws of gravity change at very low accelerations (i.e., on very large distance scales).
*A simulation of the formation of dark matter structures from the early Universe until today. Credit: Ralf Kaehler/SLAC National Accelerator Laboratory/AMNH*
In addition, astronomers have long held that there is a simple relationship between the amount of visible (baryonic) matter a galaxy contains and the gravitational force it exerts – known as the Radial Acceleration Relation (RAR). While this theory certainly applies to larger systems, the new study suggests that it breaks down in the smallest galaxies. Upon examining 12 dwarf galaxies and inferring their mass distributions, they found that MOND predictions failed to reproduce the observed behavior, proving that their gravitational fields could not be explained by visible matter alone.
They then compared their results with theoretical models that assume the presence of dark matter haloes around galaxies using the DiRAC National Supercomputer facility. The results of these simulations provided a much better match for the observed behavior of these dwarf galaxies. According to Mariana Júlio, a PhD student at the AIP and the lead author of the study:
The smallest dwarf galaxies have long been in tension with MOND predictions, but the discrepancy could plausibly be explained by measurement uncertainties, or by adapting the MOND theory. For the first time, we were able to resolve the gravitational acceleration of stars in the faintest galaxies at different radii, revealing in detail their internal dynamics. Both the observations and our EDGE simulations show that their gravitational field cannot be determined by their visible matter alone, contradicting modified gravity predictions. This finding reinforces the need for dark matter and brings us closer to understanding its nature.
The study challenges the RAR paradigm by providing better and more in-depth analysis, allowing astronomers to properly infer the radially resolved profiles of dwarf galaxies. They further confirm what astronomers suspected about dwarf galaxies and how they do not conform to the expectations of their more massive counterparts. Said co-author Professor Justin Read from the University of Surrey:
New data and modelling techniques are allowing us to map out the gravitational field on smaller scales than ever before, and this is giving us new insights into the strange, apparently invisible, substance that makes up most of the mass of the Universe. Our results demonstrate that there is not enough information based only on what we can see to determine the gravitational field strength in the smallest galaxies. This result can be explained if these galaxies are surrounded by an invisible halo of dark matter, as the dark matter encodes the ‘missing information’. But MOND theories – at least those proposed so far – require the gravitational field to be determined only by what we see. That just doesn’t seem to work.
While the findings do not address the outlying questions about DM (e.g., what it’s composed of) or confirm its existence, they do narrow the search by helping to rule out alternative explanations. Future observations that target even fainter and more distant galaxies will further narrow the search, and scientists will do so with confidence that DM is still the most likely explanation for what we see out there.
Further Reading: Leibniz Institute for Astrophysics Potsdam, arXiv