DR. SONALI SACHDEVA

Increasing amount of observational evidence suggests that z~2 marks a critical junction in the evolutionary history of galaxies. This is the epoch at which cosmic star formation density peaks, leading to shorter gas depletion time-scales and correspondingly higher gas accretion rates for replenishment. In addition to that, merger frequency peaks at around this epoch, especially for low mass galaxies. Crucially, structure of the galaxies is observed to differ remarkably on the two sides of z~2. While Hubble sequence is reported to be nearly in place by z~1.2-1.5, almost all galaxies at redshifts higher than 2.5-3 are found to be irregulars. To decipher the emergence of the Hubble sequence, i.e., understand the internal and external mechanisms involved in the formation of discs and spheroids, we focus on a sample which has most accurate measurements of redshifts and stellar parameters for z>1.5. Performing structural decomposition of each galaxy in this sample individually, we carefully examine the change in structure and corresponding stellar properties of galaxies on the two sides of redshift 2. The speaker will present the results of this analysis in the talk which strongly suggest that z~2 is majorly a disc formation period.