30th Annual Report 2017-18

as the Saraswati within the current age of the Universe. Thus, discovery of this extremely large cosmic structure forces astronomers into re-thinking the popular theories of how the Universe got its current form, starting from a more-or-less uniform distribution of energy after the Big Bang. It is believed that galaxies are formed mostly on the filaments and sheets that are part of the cosmic web, andmany of the galaxies travel along these filaments, ending up in the rich clusters, where the crowded environment switches off their star formation and aids in the transformation of galaxies from disky blue spiral galaxies to red elliptical galaxies. Since there is an extensive variation of environment within a supercluster, galaxies travel through these varied environments during their lifetime. To understand their formation and evolution, one needs to identify these superclusters and closely study the effect of their environment on the galaxies. This is a very new research area, andwith the aid of new observational facilities, astronomers are now beginning to understand galaxy evolution. The discovery of Saraswati will greatly enhance this field of research. The researchers were very surprised to spot this giant wall-like supercluster of galaxies, clearly visible in a large spectroscopic survey of distant galaxies (the SDSS). This supercluster is clearly embedded in a large network of cosmic filaments traced by clusters and large voids. Previously only a few comparatively large superclusters have been reported, for ex- ample, the nearby 'Shapley Concentration' (discovered by Somak Raychaudhury in 1989) or the 'Sloan Great Wall' in the nearby Universe, while the Saraswati supercluster is far more distant one. Our own galaxy is part of a smaller supercluster called the Laniakea . The discovery of Saraswati supercluster will help to shed light on the perplexing question; how such extreme large scale, prominent matter-density enhancements had formed billions of years in the past when the mysterious Dark Energy had just started to dominate structure formation? Although the Saraswati supercluster is around 650 million light years in one direction, its extent in the transverse direction on the sky is not completely known because of the limited data currently available. It is strongly suspected that Saraswati supercluster may have far more extension than what is suggested by the currently available sparse data from the SDSS. This discovery is now being followed up by the original team and their collaborators, involving many more researchers from Indian Universities. In-depth studies are being carried out at optical, radio and X-ray wavelengths, using many international facilities, including Indian telescopes like the Giant Metrewave Radio Telescope (GMRT), at Khodad, near Pune, the Indian AstroSat satellite in space (launched and maintained by ISRO), 40 Figure 11: Two of the most massive galaxy clusters of the Saraswati supercluster – Left : Abell 2631 and Right : ZwCl 2341.1+0000. These two images are taken from the public Dark Energy Legacy Survey (DECaLS).