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Superclusters

The Laniakea Supercluster

Virgo Supercluster Density Map The Laniakea (Lani=Heaven + akea=immense in Hawaiian) supercluster is a more precisely defined entity than the Virgo cluster (the region enclosed inside the ellipse in Figure 03-06c). Figure 03-06d shows the 2014 update of the density map in the Virgo neighourhood (derived from the velocity measurements, red-high, green-intermediate, blue-low). The velocity flows (of the galaxies) are illustrated by the black threads, ultimately all flows appear to drain towards the Shapley supercluster. The flow pattern within Laniakea is shown in Figures 03-06e and 03-06f.

Figure 03-06c Virgo (Local) Supercluster [view large image]

Figure 03-06d Density Map [view large image]

The novel idea in defining the Laniakea involves the association of the peculiar velocity Vp with each galaxy by the formula (the observed minus the cosmic expansion velocities) :

Vp = Vo - d Ho

where Vo is the velocity obtained by red shift measurement, d is the distance, the Hubble constant Ho is taken to be 75.2 km/s-Mpc
Galactic Flow Laniakea Supercluster because it renders a neat construction of Laniakea. This value of Ho is considerably different from the value of 67.15 km/s-Mpc obtained from the latest Planck measurement and required an explanation. Anyway, for a given observed Vo , Vp = Ho d , which yields a relationship of 1000 km/s (in the velocity grids of Figure 03-06d,e,f) equivalence to 13.3 Mpc (the relative distance scale).

Figure 03-06e Galactic Flow
[view large image]

Figure 03-06f Supercluster, Laniakea [view large image]

The data are taken from the Cosmicflows-2. After separating the local and tidal velocity components, a special volume emerges in which all the flow lines point inward converging to the Great Attractor near the Norma cluster (Figure 03-06e) as observed previously; while those outside flow away. The boundary acts like the watershed dividing the flows to inward and outward directions. Figure 03-06f displays the same Laniakea supercluster with shaded contours representing density values (red-high, green-intermediate, blue-void). The dark streamlines represent velocity flows outside Laniakes, and the dark shade indicates the swath of the obscuration in the plane of the Milky Way.

The density is computed from the divergence of the velocity field Vp related to sources and sinks :

Vp = -Hof(m,)

where f(m,) is a function of the cosmic mass m (dark+brayonic) and dark energy parameters respectively. A casual inspection of the density profile in Figure 03-06f shows that the sources of the velocity flows are from the lower density region with sinks in the higher density ridges. It seems that galaxies are attracted by some unseen entity - probably the dark matter - over there.

See "Supplementary Video" and the original paper "The Laniakea supercluster of galaxies".

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