Flattened Darkness: Stellar Streams Outline Invisible Halos
Original: "Constraints on the population level distribution of nearby Dark Matter halo shapes with extragalactic streams"
Analysis of 32 extragalactic stellar streams shows that dark matter halos around nearby galaxies are on average squashed like pumpkins.
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Stellar tails of galaxies, like ribbons of dancers, have outlined the shape of dark matter halos. It turns out invisible matter is not spherical — it's squashed like a pumpkin. Analysis of 32 streams showed an average flattening of about 0.72, perfectly matching cosmological simulations. Soon surveys like Euclid will make mapping the hidden universe routine.
🎯 For the 'golden' sample of 17 streams, the average flattening was found to be 0.72 — as if the halo is squashed by a quarter compared to a sphere.
\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.