A Council of Giants

Abstract

It is demonstrated that the Local Sheet is both geometrically and dynamically distinct from the Local Supercluster and that the evolution of the Sheet and Local Group were probably interconnected. The Sheet is inclined by 8 degrees with respect to the Local Supercluster, and the dispersion of giant members about the mid-plane is only 230 kpc. A "Council of Giants" with a radius of 3.75 Mpc encompasses the Local Group, demarcating a clear upper limit to the realm of influence of the Local Group. The only two giant elliptical galaxies in the Sheet sit on opposite sides of the Council, raising the possibility that they have somehow shepherded the evolution of the Local Group. The position vector of the Andromeda galaxy with respect to the Milky Way deviates only 11 degrees from the Sheet plane and only 11 degrees from the projected axis of the ellipticals. The Local Group appears to be moving away from a ridge in the potential surface of the Council on a path parallel to the elliptical axis. Spin directions of the giants in the Council are distributed over the sky in a pattern which is very different from that of giants beyond, possibly in reaction to the central mass asymmetry that developed into the Local Group. By matching matter densities of Group and Council giants, the edge of the volume of space most likely to have contributed to the development of the Local Group is shown to be very close to where gravitational forces from the Local Group and the Council balance. The boundary specification reveals that the Local Sheet formed out of a density perturbation of very low amplitude (~10%), but that normal matter was incorporated into galaxies with relatively high efficiency (~40%). It appears that the development of the giants of the Local Sheet was guided by a pre-existing flattened framework of matter.

26 pages, 6 figures, 5 tables. MNRAS (2014), Advance Access. First published online: March 10, 2014

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