Astrophysics has long pursued the enigmatic concept of dark matter. This investigation was notably advanced by Vera Rubin in the 1970s when it became apparent that the outer regions of galaxies rotate more rapidly than visibility would suggest. Researchers categorized this occurrence under the umbrella of dark matter. Observations such as how light bends around galaxy clusters and the distribution of matter across the universe, alongside fluctuations in the cosmic microwave background radiation, all indicate that a substantial portion of the universe remains unseen.
Current cosmological models, particularly the ΛCDM framework, suggest that dark matter consists of slow-moving particles possessing mass and gravitational influence but negligible electromagnetic interaction. This makes dark matter virtually invisible and capable of traversing through ordinary matter.
The ongoing search for dark matter particles aims to elucidate their properties and distribution within the Milky Way galaxy. While scientists can calculate the motion of stars from the galactic center to the sun without acknowledging dark matter, the dynamics shift beyond this range. A dark matter halo envelops the galaxy, extending approximately 230,000 parsecs or 4 quintillion miles (7 quintillion kilometers) from the center, and is believed to constitute about 95% of the galaxy’s total mass.
A research team from University College London explored the geometry of the Milky Way’s dark matter halo. They assumed the galaxy was in equilibrium and examined stable star positions at the galaxy’s outskirts to model the shape and orientation of the dark matter halo necessary for these arrangements. By aligning this model with historical data on the Milky Way’s development, they gained deeper insights into the galaxy’s structure.
Utilizing the Gaia survey—a satellite mission mapping millions of stars in the Milky Way from 2013 to 2025—the team analyzed the average number of stars in the galaxy’s older outer regions, referred to as the stellar halo. They also assessed the position and velocity of stars within it, discovering that the stellar halo is elliptical and tilted relative to the Milky Way due to a similarly shaped but significantly larger dark matter halo.
A simplified diagram illustrating the shape and orientation of the dark matter halo compared to the stellar halo and the Milky Way’s disk. Not to scale. By the author.
The research team concluded that their findings challenge previous models suggesting the dark matter halo is almost spherical. They determined that the halo’s tilt relative to the Milky Way’s disk is approximately 43°. This tilt is comparable to that of other disk galaxies with dark matter halos, which average about 46.5° and exhibit a 18° greater inclination than stellar halos. They posited that a stable, tilted, non-spherical dark matter halo implies overall galaxy stability, especially given its collision with another galaxy at least 8 billion years ago. Enhanced measurements of the halo’s shape could yield further insights into this merger.
For future research endeavors, the team developed a model representing a snapshot of a galaxy with a tilted, rectangular dark matter halo, integrating the density and motion of stars. Their simulations exhibit additional nuances consistent with observations from the Gaia survey, indicating that the halo becomes increasingly tilted—with angles ranging from 10 degrees near the center to 35 degrees at distances of 6 to 60 kiloparsecs (100 to 100 quintillion miles, or 200 to 2 quintillion kilometers)—and transitions from elliptical to more circular shapes as the distance from the center increases. The team suggests that subsequent research could build on this model and explore more intricate features, such as interactions between the Milky Way and neighboring galaxies including the Large Magellanic Cloud.
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Source: sciworthy.com
