A New Way to Measure Where the Milky Way’s Dark Matter Is

Our Milky Way Galaxy is rich in dark matter. The problem is, we can’t see where it’s distributed because, well, it’s dark. We also don’t completely understand how it’s distributed—in clumps or what? A team at the University of Alabama-Huntsville has figured out a way to use solitary pulsars to map this stuff and unveil its effect on the galaxy.

A technique developed by Dr. Sukanya Chakrabarti and her team is based on some unique characteristics of pulsars. In addition, it uses the presence of a strange wobble of our galaxy. It seems to be induced by interactions with dwarf galaxies such as the Large Magellanic Cloud. That wobble has a connection to the amount of dark matter in the galaxy, and it turns out that pulsars can help map it.

Dark Matter Mapping and Pulsars

Pulsars are the corpses of massive stars. After they explode as supernovae, what remains is a rapidly spinning compressed stellar core. These beasts sport incredibly strong magnetic fields. Those fields twist and coil up as they spin many times per second and send high-speed particles out to space. That causes the pulsar to lose energy. Combined with friction produced by the motions of the twisted magnetic field, the pulsar slows down ever so slightly in a process called “magnetic braking”. Scientists have worked for years to model this process to understand the behavior of pulsars.

The Milky Way Galaxy’s behavior is another part of the dark matter mapping puzzle. Astronomers know it has a substantial component of dark matter that appears not to be evenly spread out. The actual distribution of that mass of dark matter leads to some interesting effects, according to Chakrabarti. “In my earlier work, I used computer simulations to show that since the Milky Way interacts with dwarf galaxies, stars in the Milky Way feel a very different tug from gravity if they’re below the disk or above the disk,” she said. “The Large Magellanic Cloud (LMC)–a biggish dwarf galaxy–orbits our own galaxy, and when it passes near the Milky Way, it can pull some of the mass in the galactic disk towards it–leading to a lopsided galaxy with more mass on one side, so it feels the gravity more strongly on one side.”

Chakrabarti compared this interesting galaxy “wobble” to the way a toddler walks–not entirely balanced yet. That wobble affects stars, including pulsars. And it turns out that the different tugs of gravity caused by the distribution of dark matter affects their spindown rates. “So this asymmetry or disproportionate effect in the pulsar accelerations that arises from the pull of the LMC is something that we were expecting to see,” said Chakrabarti. In other words, those tugs of gravity by dark matter give away its distribution and possibly its density throughout the Galaxy.

Building on Previous Work

Chakrabarti and her team previously pioneered the use of binary pulsars to map dark matter in the Galaxy. It turns out that magnetic braking doesn’t affect the orbits of pulsars in binary systems. That makes them useful to measure the amount and distribution of dark matter in the Milky Way. So, the team measured the acceleration of pulsar spin rates due to the effect of the Milky Way’s gravitational potential. That work showed it’s possible to map the galaxy’s gravitational field with data points from more binary pulsars. That includes clumps of galactic dark matter. However, there’s a problem. There are a lot of singular pulsars. There had to be a way to use them, too. And that brings us back to the team’s modeling of pulsar spindown.

“Because of this spindown, we were initially–in 2021 and in our follow-up 2024 paper–forced to use only pulsars in binary systems to calculate accelerations because the orbits aren’t affected by magnetic braking,” said team member Tom Donlon. “With our new technique, we are able to estimate the amount of magnetic braking with high accuracy, which allows us to also use individual pulsars to obtain accelerations.”

Need More Data

Adding more “point source” measurements with single pulsars, Chakrabarti’s team predicts that it should eventually be possible to determine a much more accurate understanding of the distribution of dark matter in the Milky Way. “In essence, these new techniques now enable measurements of very small accelerations that arise from the pull of dark matter in the galaxy,” Chakrabarti said. “In the astronomy community, we have been able to measure the large accelerations produced by black holes around visible stars and stars near the galactic center for some time now. We can now move beyond the measurement of large accelerations to measurements of tiny accelerations at the level of about 10 cm/s/decade. 10 cm/s is the speed of a crawling baby.”

For More Information

UAH Breakthrough Enables the Measurement of Local Dark Matter Density Using Direct Acceleration Measurements for the First Time
Empirical Modeling of Magnetic Braking in Millisecond Pulsars to Measure the Local Dark Matter Density and Effects of Orbiting Satellite Galaxies
Galactic Structure From Binary Pulsar Accelerations: Beyond Smooth Models

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