Dr. Sukanya Chakrabarti, the Pei-Ling Chan Endowed Chair in the College of Science at The University of Alabama in Huntsville.
Michael Mercier | UAH
Dr. Sukanya Chakrabarti, the Pei-Ling Chan Endowed Chair in the College of Science at The University of Alabama in Huntsville (UAH), and her team have published a new paper that for the first time uses binary and solitary pulsars to constrain properties of a dark matter sub-halo in our own galaxy. Sub-halos are smaller clumps of dark matter that reside within a larger dark matter halo – regions of invisible matter surrounding galaxies and galaxy clusters known only through their gravitational effects.
“Imagine the galaxy as a cupcake, and the dark matter sub-halos are like chocolate chips on top of the cupcake,” Chakrabarti explains. “The galaxy – without the chocolate chips – is pretty smooth looking. The dark matter sub-halos contribute an additional signal on top of the smooth galactic component that we can now detect.”
The research builds on earlier work at UAH, a part of The University of Alabama System, helping to determine how much of this mysterious substance is in the Milky Way and where it is located.
Sub-halos are smaller clumps of dark matter that reside within a larger dark matter halo.
Courtesy NASA
“Our central goal in using pulsar accelerations was always to understand the nature of dark matter. These dark sub-halos are the lynchpin of dark matter models, and we now think we have a means of finding them,” the researcher notes. “Our determination of the mass of this dark matter sub-halo is much more precise than any previous method.”
Dark matter halos – thought to be the underlying structure on which galaxies are built – are important to understanding galaxy formation and evolution. The current theory of structure formation in the universe predicts that dark matter sub-halos should be abundant in galaxies like the Milky Way, yet finding them has proved challenging.
“Our localization is now pretty good in all three coordinates, and future acceleration measurements will improve the mass significance even further,” Chakrabarti says. “In any case, it is still more precise than anything that has been done before.”
“Localization” refers to identifying specific regions within a larger structure where dark matter effects are more pronounced. The concept is crucial for distinguishing potential dark matter interactions from background noise and understanding how dark matter interacts with regular matter and shapes the universe.
According to the study, these characteristics are obtained by “analyzing, for the first time, the excess, correlated power in the acceleration field of binary pulsars,” a phenomenon where the observed accelerations exhibit a pattern that deviates from what is expected based on Newtonian gravity and known astrophysical sources.
“The ‘excess’ power is basically the chocolate chips – the sub-halos – that stand out from the cupcake,” the researcher says. “By power we are referring to the acceleration signal – the dark matter sub-halos contribute an additional signal on top of the smooth galactic component that we can now detect. By ‘correlated,’ what we mean is that the signal has been experienced by pairs of pulsars. This is a more stringent requirement than requiring that an excess signal is experienced by one pulsar alone.”
Advancing this kind of research depends on the availability of precise binary pulsar acceleration data.
“In our very first work back in 2021, we didn't have enough pulsars to do this – we could only measure the smooth component of the potential,” Chakrabarti says. Smooth components in a galaxy feature a relatively uniform and diffuse distribution of stars and gas, presenting a largely undisturbed distribution of dark matter within a galaxy's halo.
“But as our sample continued to grow, it became clear that pretty soon we would be able to measure these dark matter clumps directly. As we get more sensitive observations in the future, we will be able to do this analysis to find dark matter sub-halos far beyond the solar neighborhood as well,” the researcher says. “Ultimately, these future observations will let us discriminate between models of dark matter.”
Characterizing sub-structure is key to understanding and ultimately pinning down the nature of dark matter, because the various models differ in how these clumps are distributed. Such advances hold the promise of supplanting other dark matter models.
Looking to the future, Chakrabarti and her colleagues have demonstrated that this work is a significant step toward at last illuminating one of the great mysteries of the universe.
“I think the next step is to increase our samples of precise accelerometers so that we can get more detections – that are also more precise – of dark matter sub-halos,” she says. “That will ultimately enable us to clearly discriminate between dark matter models and determine the nature of dark matter, which is one of the outstanding problems in astronomy and has been for the last century.”