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A minor merger origin for stellar inner discs and rings in spiral galaxies

Recent observations show that inner discs and rings (IDs and IRs, henceforth) are not preferably found in barred galaxies, a fact that points to the relevance of formation mechanisms different to the traditional bar-origin scenario. In contrast, the role of minor mergers in the formation of these inner components (ICs), while often invoked, is still poorly understood. We have investigated the capability of minor mergers to trigger the formation of IDs and IRs in spiral galaxies through collisionless N-body simulations. We have run a battery of minor mergers in which both primary and secondary are modelled as disc-bulge-halo galaxies with realistic density ratios. A detailed analysis of the morphology, structure, and kinematics of the ICs resulting from the minor merger has been carried out. All the simulated minor mergers develop thin ICs out of satellite material, supported by rotation. A wide morphological zoo of ICs has been obtained (including IDs, IRs, pseudo-rings, nested IDs, spiral patterns, and combinations of them), but all with structural and kinematical properties similar to observations. The existence of the resulting ICs can be deduced through the features that they imprint in the isophotal profiles and kinemetric maps of the final remnant, as in many real galaxies. The realistic density ratios used in the present models make the satellites to experience more efficient orbital circularization and disruption than in previous studies. Combined with the disc resonances induced by the encounter, these processes give place to highly aligned co- and counter-rotating ICs in the remnant centre. Therefore, minor mergers are an efficient mechanism to form rotationally-supported stellar ICs in spiral galaxies, neither requiring strong dissipation nor the development of noticeable bars (abridged).