Faint Light Reveals the Shape of Invisible Dark Matter ⚡ экспресс
The possibility of using intracluster light (ICL) as an indicator of the shape of dark matter (DM) halos in galaxy clusters has been investigated. In the Hydrangea cosmological simulations, 3D and projected shapes of both components were measured using the tensor method in ellipsoidal shells out to the virial radius. The axes of ICL and DM nearly coincide (deviation <10°), and the axis ratios differ on average by only 0.07 (DM is slightly rounder). These trends are consistent in 2D/3D and are confirmed by isophotal analysis of mock images. The axis differences are influenced by the satellite removal method and possibly by subgrid physics. ICL traces the DM shape better than satellite galaxies (whose distribution is more elliptical and has larger scatter). Therefore, intracluster light is a reliable indicator of the ellipticity and orientation of dark matter halos.
In galaxy clusters, invisible dark matter serves as the framework holding star systems together. Its shape is betrayed by a faint glow—stars ripped from galaxies by tidal forces. This stellar 'broth' fills the cluster like a liquid taking the shape of its container: it envelops the clumps of dark matter, creating a precise cast of their outlines. Astronomers measured the elongation of the light cloud and compared it to the shape of the dark halo: the direction of flattening matches to within ten degrees, and the contours themselves differ by only a few percent. Astonishingly, this glow is made of billions of stars—yet together they shine fainter than a large galaxy, less than one percent of its brightness. The method uses photometry—a precise measurement of brightness that reveals faint details. It provides a more reliable picture than previous estimates based on galaxy motions. Back in the 1930s, Fritz Zwicky suspected invisible mass in clusters; now its outlines can be read in the silent light.
🎯 Though billions of stars produce this light, altogether they shine at less than 1% the brightness of the cluster's central galaxy.