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Galaxies congregate in clusters and along filaments, and are missing from large regions referred to as voids. These structures are seen in maps derived from spectroscopic surveys^sup 1,2^ that reveal networks of structure that are interconnected with no clear boundaries. Extended regionswith a high concentrationof galaxies are called 'superclusters', although this term is not precise. There is, however, another way to analyse the structure. If the distance to each galaxy from Earth is directly measured, then the peculiar velocity can be derived from the subtraction of the mean cosmic expansion, the product of distance times the Hubble constant, from observed velocity. The peculiar velocity is the line-of-sight departure from the cosmic expansion and arises from gravitational perturbations; a map of peculiar velocities can be translated into a map of the distribution of matter^sup 3^. Here we report a map of structure made using a catalogue of peculiar velocities. We find locations where peculiar velocity flows diverge, as water does at watershed divides, and we trace the surface of divergent points that surrounds us. Within the volume enclosed by this surface, the motions of galaxies are inward after removal of themean cosmic expansion and long range flows. We define a supercluster to be the volume within such a surface, and so weare defining the extent of our home supercluster, which we call Laniakea.
The distribution of matter can be determined by two independent methods: either based on surveys of the distribution of galaxies in pro- jection and redshift, or from the motions of galaxies. With the former, using galaxy redshift surveys, the assumption is required that the galaxy 'lighthouses' and mass distribution are strongly correlated, a condition that requires confirmation if, as is suspected, only a minor fraction of matterisbaryonic.Moreover,withtheformer there is a stringent demand that the survey be complete, or at least that its incompleteness be well understood. With the latter, studies of galaxy motions, sparse sampling is acceptable (indeed inevitable) but dealing with errors is a challenge. Except for the very closest galaxies, uncertainties in distance measure- ments translate into uncertainties in the peculiar velocities of galaxies that are larger in amplitude than the actual peculiar velocities. Many mea- surements are requiredfor suitable averaging and care must be taken to avoid systematic errors....