30th Annual Report 2017-18

38 Large-scale structures in the Universe are found to be hierarchically assembled, with galaxies, together with associated gas, and dark matter, being clumped in clusters, which are organized with other clusters, smaller groups, filaments, sheets and large empty regions (“voids”) in a pattern called the “CosmicWeb” which spans the observable Universe. Superclusters are the largest coherent structures in the Cosmic Web. A supercluster is a chain of galaxies and galaxy clusters, bound by gravity, often stretching to several hundred times the size of clusters of galaxies, consisting of tens of thousands of galaxies. Recently, Joydeep Bagchi , Somak Raychaudhury , and Pratik Dabhade from IUCAA, and researchers from three other Indian Universities (Shishir Sankhyayan, Prakash Sarkar and Joe Jacob) have identified a previously unknown, extremely large supercluster of galaxies located in the direction of constellation Pisces. This supercluster is the first major discovery of its kind made in India involving Indian Universi- ties. This is one of the largest known structures in the observable Universe, and is at a distance of about 4,000 million light-years (at the redshift, z 0 . 28) away fromus. The supercluster is given the name ' Saraswati ' (Figure 9). We named this structure not only after the Indian Goddess of knowledge, music, art, wisdom and nature, but also after the “ever flowing river with many streams”; the ancient Saraswati river mentioned in the Rig'veda , since the structure projected on the sky has a similar appearance (it might also be mentioned that the Milky Way, the band of stars in the sky that represents the plane of our own Galaxy, is also called “Akashganga” in Indian languages). The newly-discovered Saraswati supercluster extends over an eno rmous s ca l e o f 650 mi l l i on l i gh t - yea r s ( 200 Mega parsecs), contains at least 48 individual galaxy clusters and groups and may contain the mass equivalent of over 16 20 million billion Suns ( 2 × 10 M ). When astronomers look far away, they see the Universe as it was long ago, since light takes a while to reach us. The Saraswati supercluster is one of the farthest supercluster ever observed, appearing as it was when the Universe was about 10 billion years old. A major concentration of galaxies, comprising the Saraswati supercluster, was identified using the friends-of-friends (FoF) algorithm from the vast spectroscopic galaxy data of the Sloan Digital Sky Survey (SDSS) in the Stripe-82 region. In an FoF algorithm, a linking length ( l ) is chosen for the galaxy distribution. If the separation between two galaxies is less than or equal to l , these two galaxies are considered linked and part of the same overdense structure, otherwise not. Defined in this manner the size of a supercluster naturally becomes sensitive to the linking length used to determine it. The l for a given galaxy distribution is generally chosen such that maximum number of Figure 9: The distribution of galaxies ( 3000), from Sloan Digital Sky Survey (SDSS), in and around Saraswati supercluster. The wedge shown has ◦ ◦ ◦ ◦ 336 ≤ R.A. ≤ 16 , − 1 . 25 ≤Dec. ≤ +1 . 25 and 0 . 23 ≤ z ≤ 0 . 33, which is centered on the Saraswati supercluster. It is clearly seen that the density of galaxies is very high in the central region of the image where the supercluster is located. It is surrounded by a cosmic-web of huge voids, filaments and clusters. DISCOVERY OF SARASWATI SUPERCLUSTER