What is the diameter of this mass concentration of galaxies that contains the Local Sheet?
Local Sheet The Local Sheet in astronomy is a nearby region of the observable universe where the Milky Way, the members of the Local Group and other galaxies share a similar peculiar velocity. This region lies within a radius of about 7 Mpc , 1.5 Mly thick, and galaxies beyond that distance show markedly different velocities. The Local Group has only a relatively small peculiar velocity of with respect to the Local Sheet. Typical velocity dispersion of galaxies within the Local Sheet is only in the radial direction. All nearby bright galaxies belong to the Local Sheet. The Local Sheet is part of the Local Volume and is in the Virgo Supercluster (Local Supercluster).
Virial mass In the context of dark matter halos of galaxies or galaxy clusters, virial mass is the mass within the virial radius formula_1, a radius within which a spherical "top hat" density perturbation destined to become a galaxy is collapsing. This radius is defined as where formula_2 where formula_3 is the halo's average density within that radius, formula_4 is a constant, and formula_5 is the critical density of the universe. (Sometimes in the definition, formula_5 is replaced with the mean density of matter, formula_7, where, at the present day, formula_8 according to data fitted to the Lambda-CDM model.) formula_4 depends on the cosmological model. In an Einstein-de Sitter model it equals formula_10 The virial mass is the mass within this radius and hence is a reasonable measure of the total mass inside a dark matter halo, because beyond that radius the halo blends into the background matter in the universe. This definition is not universal, however, as the exact value of formula_4 depends on the cosmology – in practice, to improve communication, it is sometimes simply assumed that formula_12 and this is signaled by denoting the virial mass as formula_13 and the virial radius as formula_14