DR. SONALI SACHDEVA

The presence of pure disc galaxies without any bulge component, i.e., neither classical nor pseudo, poses a severe challenge not just to the hierarchical galaxy formation models but also to the theories of internal secular evolution. We discover that a significant fraction of disc galaxies (~ 15-18 %) in the Hubble Deep Field as well as in the local Universe are such pure disc systems (PDS). We trace the evolution of this population to find how they have survived the merger violence and other disc instabilities to remain dynamically undisturbed. We find that smooth accretion of cold gas via cosmic filaments is the most probable mode of their growth since z~1. We speculate that PDSs are dynamically hotter and cushioned in massive dark matter halos which prevents them from undergoing strong secular evolution.