Stellar Streams Reveal the Shape of Invisible Halos

Original: "Constraints on the population level distribution of nearby Dark Matter halo shapes with extragalactic streams"

· David Chemaly, Elisabeth Sola, Sergey Koposov, HanYuan Zhang, Vasily Belokurov, Denis Erkal

arXiv:2607.05510v1 · 2026-07-06 · CC BY · ⏱ 2 min · Galaxies

Analysis of stellar streams beyond our Galaxy shows that dark matter doesn't form spheres, but slightly squashed shapes.

Abstract

Stellar streams are like glowing rivers winding around invisible mountains of dark matter. By studying the bends in such streams in distant galaxies, scientists found that dark halos are squashed like a pumpkin. This helps us understand how the Universe is structured. What would we see if we could peek behind the veil of darkness?

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Around galaxies, there are clouds of dark matter — invisible substance that doesn't glow but pulls on everything around it. Astronomers look at stellar streams — long strings of stars that were once part of destroyed satellite galaxies. These streams stretch across the sky like galactic 'crumbs,' tracing the path of ancient orbits. They are like trickles of paint skirting invisible obstacles. From the bends in these trickles, we can infer the shape of what distorts them — the shape of the dark matter halo.

Dark matter makes up about 85% of all matter in the Universe, but to this day, nobody knows exactly what it's made of.

Scientists used data from the STRRINGS catalog, which contains 32 such streams around nearby galaxies. Through complex calculations that compare real arcs with computer models, they found: on average, the halos are squashed like a pumpkin, with an average thickness of 0.72 relative to their diameter. Perfectly round halos are almost ruled out. This work was started by pioneers — Fritz Zwicky first suspected hidden mass, and Vera Rubin proved its influence on galaxy rotation. Edwin Hubble discovered the world of galaxies itself. Now we see that their invisible frameworks are not perfect spheres.

The halo shape affects experiments searching for dark matter particles on Earth: if it's squashed, its density near us could differ from the round version.

This shape aligns with supercomputer simulations that account not only for dark matter but also for ordinary gas and stars. This is important for the entire standard cosmological model: our picture of the Big Bang and the expansion of the Universe with dark energy gets another confirmation. Just as entropy in nature tends toward disorder, dynamical processes smooth out the shape of halos over time. And methods akin to analyzing gravitational lensing allow us to 'see' the invisible. In the future, new sky surveys will add thousands of streams, and we will learn much more about dark matter.

🎯 There is five times more dark matter in space than ordinary matter—the very stuff that makes up people, planets, and distant stars.

\log P(D|\theta) = -\frac{1}{2}\sum_{i=1}^N \left[ \frac{(r_{\rm model}^{i} - r_{\rm data}^{i})^2}{\sigma_{{\rm data},i}^2 + \sigma_{\rm sys}^2} + \log(2\pi(\sigma_{{\rm data},i}^2 + \sigma_{\rm sys}^2)) \right]

Extended likelihood function comparing model and observed data. An overall systematic dispersion σ_sys is added to the instrumental errors, itself inferred from the data to compensate for model imperfections.

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Stellar Streams Reveal the Shape of Invisible Halos