Within it, the Milky Way and Andromeda are encircled by two large galaxies arranged in a ring about 24 million lightyears across. McCall dubs this the Council of Giants, which in his words, "stands in gravitational judgment of the Local Group by restricting its range of influence." His work shows that the Local Sheet is both geometrically and dynamically distinct from the Local Supercluster (a large group of galaxies and galactic groups even further out).

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Of the 14 giants in the Local Sheet, 12 are spiral galaxies. The remaining two, which sit on opposite sides of the Council, are elliptical galaxies. McCall believes that winds expelled in the earliest phases of their formation may have shepherded gas towards the Local Group, contributing to the disks of the Milky Way and Andromeda.

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Quite unexpectedly, the spin of the Council giants are arranged around a small circle — an unusual alignment likely caused by gravitational torques imposed by our galaxy and Andromeda when the universe was smaller.

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Into the Void

Interestingly, the outer boundary of the Council is helping astronomers understand the conditions which led to the formation of the Milky Way. To create such an orderly configuration as the Local Sheet and its Council, nearby galaxies likely developed within a pre-existing sheet-like foundation made primarily from dark matter.

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"Recent surveys of the more distant universe have revealed that galaxies lie in sheets and filaments with large regions of empty space called voids in between," noted McCall in a statement. "The geometry is like that of a sponge. What the new map reveals is that structure akin to that seen on large scales extends down to the smallest."

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McCall's study appears in the Monthly Notices of the Royal Astronomical Society.

Images: Marshall McCall / York University.

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