The Argo simulation - II. The early build-up of the Hubble sequence

The Hubble sequence is a common classification scheme for the structure of galaxies. Despite the tremendous usefulness of this diagnostic, we still do not fully understand when, where, and how this morphological ordering was put in place. Here, we investigate the morphological evolution of a sample of 22 high-redshift (z >= 3) galaxies extracted from the Argo simulation. Argo is a cosmological zoom-in simulation of a group-sized halo and its environment. It adopts the same high-resolution (similar to 10(4) M-circle dot, similar to 100 pc) and sub-grid physical model that was used in the Eris simulation but probes a sub-volume almost 10 times bigger with as many as 45 million gas and star particles in the zoom-in region. Argo follows the early assembly of galaxies with a broad range of stellar masses (log M-star/M-circle dot similar to 8-11 at z similar or equal to 3), while resolving properly their structural properties. We recover a diversity of morphologies, including latetype/irregular disc galaxies with flat rotation curves, spheroid dominated early-type discs, and a massive elliptical galaxy, already established at z similar to 3. We identify major mergers as the main trigger for the formation of bulges and the steepening of the circular velocity curves. Minor mergers and non-axisymmetric perturbations (stellar bars) drive the bulge growth in some cases. The specific angular momenta of the simulated disc components fairly match the values inferred from nearby galaxies of similar M-star once the expected redshift evolution of disc sizes is accounted for. We conclude that morphological transformations of high-redshift galaxies of intermediate mass are likely triggered by processes similar to those at low redshift and result in an early build-up of the Hubble sequence.