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).
Hierarchical models predict that present-day massive early-type galaxies (mETGs) have finished their assembly at a quite late cosmic epoch (z~0.5), conflicting directly with galaxy mass-downsizing. In Eliche-Moral et al. (2010), we presented a semi-a
nalytical model that predicts the increase by a factor of ~2.5 observed in the number density of mETGs since z~1 to the present, just accounting for the effects of the major mergers strictly-reported by observations. Here, we describe the relative, coordinated role of wet, mixed, and dry major mergers in driving this assembly. Accordingly to observations, the model predicts that: 1) wet major mergers have controlled the mETGs buildup since z~1, although dry and mixed mergers have also contributed significantly to it; 2) the bulk of this assembly takes place during the ~1.4 Gyr time-period elapsed at 0.7<z<1, being nearly frozen at z<~0.7; 3) this frostbite can be explained just accounting for the observational decrease of the major merger fraction since z~0.7, implying that major mergers (and, in particular, dry events) have contributed negligibly to the mETGs assembly during the last ~6.3 Gyr; and 4) major mergers are responsible for doubling the stellar mass at the massive-end of the red sequence since z~1. The most striking model prediction is that at least ~87% of the mETGs existing at z~1 are not the passively-evolved, high-z counterparts of present-day mETGs, but their gas-poor progenitors instead. This implies that <~5% of present-day mETGs have been really in place since z~1. The model derives a redshift of final assembly for present-day mETGs in agreement with hierarchical models (z~0.5), reproducing at the same time the observed buildup of mETGs at z<~1.(Abridged)
Several studies have tried to ascertain whether or not the increase in abundance of the early-type galaxies (E-S0as) with time is mainly due to major mergers, reaching opposite conclusions. We have tested it directly through semi-analytical modelling
, by studying how the massive early-type galaxies with log(M_*/Msun)>11 at z~0 (mETGs) would have evolved backwards-in-time, under the hypothesis that each major merger gives place to an early-type galaxy. The study was carried out just considering the major mergers strictly reported by observations at each redshift, and assuming that gas-rich major mergers experience transitory phases of dust-reddened, star-forming galaxies (DSFs). The model is able to reproduce the observed evolution of the galaxy LFs at z<~1, simultaneously for different rest-frame bands (B, I, and K) and for different selection criteria on color and morphology. It also provides a framework in which apparently-contradictory results on the recent evolution of the luminosity function (LF) of massive, red galaxies can be reconciled, just considering that observational samples of red galaxies can be significantly contaminated by DSFs. The model proves that it is feasible to build up ~50-60% of the present-day mETG population at z<~1 and to reproduce the observational excess by a factor of ~4-5 of late-type galaxies at 0.8<z<1 through the coordinated action of wet, mixed, and dry major mergers, fulfilling global trends that are in general agreement with mass-downsizing. The bulk of this assembly takes place during ~1 Gyr elapsed at 0.8<z<1. The model suggests that major mergers have been the main driver for the observational migration of mass from the massive-end of the blue galaxy cloud to that of the red sequence in the last ~8 Gyr.(Abridged)
