30th Annual Report 2017-18

39 overdense structures are obtained given a predetermined limit to the minimum number of galaxies required in an overdense structure. In the currently accepted cosmological model, clusters of galaxies trace the local extrema in the underlying distribution of matter - both dark and luminous, and since Saraswati is particularly a vast and overdense structure of galaxies, some clusters or groups of galaxies should be found at the highest density peaks. Moreover, in the cosmic web overdense regions are surrounded by almost empty voids. For quantifying more objectively, the galaxy distribution in overdense (clusters/groups, filaments) and the underdense (voids) regions of the cosmic web around the Saraswati supercluster, we crossmatch the galaxy distribution with the published catalogues of clusters and voids derived from SDSS data, and, indeed, 43 rich clusters or groups (fromWHLcluster catalogue) are found in the Saraswati supercluster, along with large voids (from Nadathur void catalogue) which surround the supercluster (Figure 10). This shows that the Saraswati supercluster is real structure and not just an artifact of the data. Out of the 43 clusters in Saraswati , the two most massive ones are theAbell 2631 and ZwCl 2341.1+0000, shown in Figure 11. The widely-accepted “cold dark matter” cosmological model of the Universe predicts that small structures like galaxies form first, which congregate into larger structures later. Most forms of this model do not predict the existence of structures as large Figure 10: Upper panel : The 3D distribution of the wall-like structure of the Saraswati supercluster, mainly comprising 43 known clusters of galaxies, here represented by spheres. The radius of each cluster is proportional to its virial radius. Colours represent the masses of the clusters. Lower panels : Overdense structures of galaxies (each dot represents a galaxy) identified in the SDSS galaxy data using Friends-of-Friends algorithm. Different colours indicate various overdense structures. The largest overdensity of galaxies found is Saraswati, spanning 200 Mpc across (in blue colour at the centre) with hundreds of galaxies. Gray dots are galaxies that are not part of any overdense structures. Lower Left : Known galaxy clusters/groups fromWHL catalogue are shown by stars. Ahigh concentration of clusters near the Saraswati supercluster is very clear. Lower Right : Giant Voids identified in the Saraswati supercluster region. Red dashed circles are voids in projection, black crosses are void centres, and the radii of circles are equal to the effective radii of voids.