AR_final file_2018-19

GALAXIES Over the years, astronomical measurements have led to the conclusion that nearly 90% of the matter in the universe is of a nature yet unknown. Called "Dark Matter" (DM) due to its near-absent interaction with electromagnetic waves, its presence is revealed only through its gravity. At IUCAA, several ongoing research programmes are aimed at developing an understanding of the nature of DM. The standard hypothesis is that the DM consists of weakly interacting massive particles, but despite long-running experiments, no evidence for such particles has yet been found. Researchers at IUCAA have explored other possibilities, one of them being Primordial Black Holes. Such black holes may have been formed in the early universe and would not be directly visible today. However, when such a black hole crosses our line of sight to a star, gravitational deflection of light would cause a temporary brightening of the star's apparent brightness, which is called a microlensing event. IUCAA astronomers have searched for microlensing of stars in our companion galaxy Andromeda, and found at most one case. The rarity of these events has led the team to conclude that Primordial Black Holes can at best account for only a small fraction of DM, the main contribution must come from some other form of material. The process of cosmological structure formation gives rise to bound structures like galaxies. Galaxies present themselves in a wide variety of shapes -- some resulting from the original assembly of material, some due to the merger of multiple early galaxies, and yet others due to the stripping of gas as the galaxy moves through a dense surrounding medium. IUCAA scientists have suggested a new mechanism for the formation of a type of galaxy called Lenticular, distinguished by a dominant, spheroidal stellar bulge. They show via numerical simulations that certain spiral galaxies with prominent disks can become unstable, causing the disk to break up, clump and contribute to the bulge formation. Structures like Galaxies and Clusters of Galaxies in the universe are thought to form by DM condensing into large self-bound Halos, the gravity of which attracts ordinary matter to flock to them and create the visible structures. The shape and density distribution of these Halos would depend sensitively on the nature of DM. While much is known about the expected properties of Halos created by the standard model of Cold Dark Matter (CDM) composed of massive leptons, IUCAA scientists have explored the properties of Halos that could be generated by an alternative form of matter composed of light Bosons, named the Fuzzy Dark Matter. Their investigations reveal a feature-rich dynamics, with Halo shells first collapsing and then expanding, leading to collisions between shells and a density distribution, quite different from that expected of CDM. Detailed observations should, in time, be able to distinguish between these two possibilities. In order to understand the progression of structure formation in the universe, one needs to resort to large numerical simulations. Given the high costs involved in carrying them out, it is a challenging task to study structures at multiple scales. Scientists at IUCAA have now devised a novel method to combine a small number of simulations of different spatial extents to accomplish this at a much reduced cost. DARK MATTER ( 39 )