Abstract
Previous studies of the galaxy and galaxy cluster distribution in the local Universe found indications for a large extension of the Local Supercluster up to a radius of 190 h70−1 Mpc. We are using our large and highly complete CLASSIX survey of X-ray luminous galaxy clusters detected in the ROSAT All Sky Survey to trace the matter distribution in the local Universe and to explore the size of the flattened local density structure associated with the Local Supercluster. The Local Supercluster is oriented almost perpendicular to the Galactic plane. Since Galactic extinction increases towards the Galactic plane, objects are on average more easily visible perpendicular to the plane than close to it, also producing an apparent concentration of objects along the Local Supercluster. We can correct for this bias by a careful treatment of the survey selection function. We find a significant overdensity of clusters in a flattened structure along the Supergalactic plane with a thickness of about 50 Mpc and an extent of about 100 Mpc radius. Structures at a distance larger than 100 Mpc are not correlated to the Local Supercluster any more. The matter density contrast of the local superstructure to the surroundings is about a factor of 1.3−2.3. Within the Supergalactic plane the matter is concentrated mostly in two superclusters, the Perseus-Pices Chain and Hydra-Centaurus supercluster. We have shown in our earlier work that the local Universe in a region with a radius of 100−170 Mpc has a lower density than the cosmic mean. For this reason, the Local Supercluster is not overdense with respect to the cosmic mean density. Therefore this local superstructure will not collapse as a whole in the future, but rather fragment.
Highlights
Our local neighbourhood in the Universe is characterised by a flattened matter density distribution which seems to show coherence over a hierarchy of scales
Since we have shown with simulations in our earlier study (Böhringer et al 2020) that clusters are true tracers of the underlying matter distribution and that Poisson statistics provides a fair account of the accuracy with which the density variation can be determined on large scales, we can draw conclusions on the matter density contrast that is involved in this cosmographical mapping
We have shown by means of the distribution of X-ray luminous clusters that the matter distribution in the local Universe shows a strong segregation towards the Supergalactic plane
Summary
Our local neighbourhood in the Universe is characterised by a flattened matter density distribution which seems to show coherence over a hierarchy of scales. Studying the density distribution of bright galaxies in the sky, notably those compiled in the Shapley-Ames galaxy survey, De Vaucouleurs noted a concentration of these galaxies towards a plane roughly perpendicular to the plane of our Galaxy (de Vaucouleurs 1953, 1956, 1958, 1959). He called this pronounced structure the ‘Local Supergalactic System’, which is referred to as the Local Supercluster. The system includes a number of galaxy groups and the Virgo cluster as its dominant member This concentration of galaxies was noted earlier by William Herschel (see Flin 1986; Rubin 1951 for a historical account). The analysis of the galaxy distribution within a radius of about 6.5 Mpc by Courtois et al (2013), for example, shows that almost all the nearby groups of galaxies, including the Local Group, the groups marked by IC 342, M 81, Cen A, M 83, N4214, and the Canes Venatici I and Maffei I groups, are contained in a very narrow slice, which is aligned with the Local Supercluster
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