We cannot see the streams of dark matter forming a web across the sky, but unusual cross-shaped galaxies may mark where they intersect.
Unlike the Milky Way鈥檚 single disc, 鈥減olar ring鈥� galaxies have two discs of stars and gas aligned nearly perpendicular to each other. It is unclear how they form, though one theory suggests they are the product of collisions between two galaxies. But galactic pile-ups are violent and messy, so are unlikely to produce the neat symmetry of a polar ring galaxy, says Fabio Governato of the University of Washington in Seattle.
Recent sky surveys suggest that most of the universe鈥檚 mass consists of clumps and filaments of invisible, flowing dark matter. Astronomers think galaxies often form at the intersection of two or more filaments, which act like colliding jets of water.
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According to a computer model built by Governato and his colleagues, material from the colliding streams would swirl and then coalesce into a galaxy within one plane. But if another filament came into play from a different direction, a second disc would take shape (). 鈥淧olar ring galaxies offer first-hand evidence of the existence of the cosmic web,鈥� he says. The team has submitted the work to The Astrophysical Journal.
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