Using three fiducial Nbody+SPH simulations, we follow the merging of two disk galaxies with a hot gaseous halo component each, and examine whether the merger remnant can be a spiral galaxy. The stellar progenitor disks are destroyed by violent relaxation during the merging and most of their stars form a classical bulge, while the remaining form a thick disk and its bar. A new stellar disk forms subsequently and gradually in the remnant from the gas accreted mainly from the halo. It is vertically thin and well extended in its equatorial plane. A bar starts forming before the disk is fully in place, contrary to what is assumed in idealised simulations of isolated bar-forming galaxies. It has morphological features such as ansae and boxy/peanut bulges. Stars of different ages populate different parts of the box/peanut. A disky pseudobulge forms also, so that by the end of the simulation, all three types of bulges coexist. The oldest stars are found in the classical bulge, followed by those of the thick disk, then by those in the thin disk. The youngest stars are in the spiral arms and the disky pseudobulge. The disk surface density profiles are of type II (exponential with downbending), and the circular velocity curves are flat and show that the disks are submaximum in these examples: two clearly so and one near-borderline between maximum and submaximum. On average, only roughly between 10 and 20% of the stellar mass is in the classical bulge of the final models, i.e. much less than in previous simulations.

We study barred galaxies selected from the Illustris cosmological simulation, focusing on tidally induced bars formed from flyby interactions. To guarantee high enough resolution we focus on high mass disc galaxies (M * > 8.3 × 10 10 M ). We find that the fraction of barred galaxies among those (21% at redshift z = 0) is lower in Illustris than observed in the local Universe, and the fraction grows slightly with redshift. The bar fraction also increases with the stellar mass and decreases with the amount of gas in the disc. Only very few bars at redshift z = 0 are formed in secular evolution (∼ 7%) and most of them are triggered by external perturbers in mergers or flybys. Many of these bars disappear over time, mostly during secular evolution, which leads to a lower fraction of bars at redshift z = 0. We then focus on the effect of flyby interactions on the disc and look at tidally induced bars created by a flyby, or pre-existing bars influenced by the passage of a perturber. In the latter case, the interaction can enhance or weaken the bar. During the interaction, the change in the bar strength occurs right after the pericentre passage. The resulting tidally induced bars tend to be stronger than the overall bar sample in Illustris. The preferred scenario to create or enhance a bar seems to be with a strong interaction involving a perturber on a prograde orbit. Furthermore, the strength of the created bar grows with the strength of the interaction.