In one widely discussed model for the formation of nuclear star clusters (NSCs), massive globular clusters spiral into the center of a galaxy and merge to form the nucleus. It is now known that at least some NSCs coexist with supermassive black holes (SBHs); this is the case, for instance, in the Milky Way. In this paper, we investigate how the presence of a SMBH at the center of the Milky Way impacts the merger hypothesis for the formation of its NSC. Starting from a model consisting of a low-density nuclear stellar disk and the SMBH, we use direct N -body simulations to follow the successive inspiral and merger of globular clusters. The clusters are started on circular orbits of radius 20 pc, and their initial masses and radii are set up in such a way as to be consistent with the galactic tidal field at that radius. These clusters, decayed orbitally in the central region due to their large mass, were followed in their inspiral events; as a result, the total accumulated mass by ≈ 10 clusters is about 1.5 × 10 7 M ⊙ . Each cluster is disrupted by the SMBH at a distance of roughly one parsec. The density profile that results after the final inspiral event is characterized by a core of roughly this radius, and an envelope with density that falls off ρ ∼ r −2 . These properties are similar to those of the Milky Way NSC, with the exception of the core size, which in the Milky Way is somewhat smaller. But by continuing the evolution of the model after the final inspiral event, we find that the core shrinks substantially via gravitational encounters in a time (when scaled to the Milky Way) of 10 Gyr as the stellar distribution evolves toward a Bahcall-Wolf cusp. We also show that the luminosity function of the Milky Way NSC is consistent with the hypothesis that 1/2 of the mass comes from old (∼ 10 Gyr) stars, brought in by globular clusters, with the other half due to continuous star formation. We conclude that a model in which a large fraction of the mass of the Milky Way is due to infalling globular clusters is consistent with existing observational constraints.

We present the results of detailed N-body simulations of clusters moving in a realistic Milky Way ( MW) potential. The strong interaction with the bulge and the disk of the Galaxy leads to the formation of tidal tails, emanating from opposite sides of the cluster. Some characteristic features in the morphology and orientation of these streams are recognized and interpreted. The tails have a complex morphology, particularly when the cluster approaches its apogalacticon, showing multiple ''arms'' in remarkable similarity to the structures observed around NGC 288 and Willman 1. Actually, the tails are generally good tracers of the cluster path quite far from the cluster center (>7Y8 tidal radii), while on the smaller scale they are mainly pointing in the direction of the Galaxy center. In particular, the orientation of the inner part of the tails is highly correlated with the cluster orbital phase and the local orbital angular acceleration. This implies that, in general, the orbital path cannot be estimated directly from the orientation of the tails, unless a sufficient large field around the cluster is available.

Nuclear Star Clusters (NSCs) are often present in spiral galaxies as well as resolved Stellar Nuclei (SNi) in elliptical galaxies centres. Ever growing observational data indicate the existence of correlations between the properties of these very dense central star aggregates and those of host galaxies, which constitute a significant constraint for the validity of theoretical models of their origin and formation. In the framework of the well known 'migratory and merger' model for NSC and SN formation, in this paper we obtain, first, by a simple argument the expected scaling of the NSC/SN mass with both time and parent galaxy velocity dispersion in the case of dynamical friction as dominant effect on the globular cluster system evolution. This generalizes previous results by Tremaine et al. (1975) and is in good agreement with available observational data showing a shallow correlation between NSC/SN mass and galactic bulge velocity dispersion. Moreover, we give statistical relevance to predictions of this formation model, obtaining a set of parameters to correlate with the galactic host parameters. We find that the correlations between the masses of NSCs in the migratory model and the global properties of the hosts reproduce quite well the observed correlations, supporting the validity of the migratory-merger model. In particular, one important result is the flattening or even decrease of the value of the NSC/SN mass obtained by the merger model as function of the galaxy mass for high values of the galactic mass, i.e. 3 × 10 11 M ⊙ , in agreement with some growing observational evidence.

In this paper we treat the problem of the dynamical friction decay of a massive object moving in an elliptical galaxy with a cuspidal inner distribution of the mass density. We present results obtained by both self-consistent, direct summation, N-body simulations, as well as by a new semi-analytical treatment of dynamical friction valid in such cuspy central regions of galaxies. A comparison of these results indicates that the proposed semi-analytical approximation is the only reliable in cuspy galactic central regions, where the standard Chandrasekhar's local approximation fails, and, also, gives estimates of decay times that are correct at 1% respect to those given by N-body simulations. The efficiency of dynamical friction in cuspy galaxies is found definitively higher than in core galaxies, especially on more radially elongated satellite orbits. As another relevant result, we find a proportionality of the dynamical friction decay time to the −0.67 power of the satellite mass, M, shallower than the standardly adopted M −1 dependence. Subject headings: s three terms in the derivative respect to M:

We have constructed realistic, self-consistent models of triaxial elliptical galaxies embedded in triaxial dark matter halos. We examined three different models for the shape of the dark matter halo: (1) with the same axis ratios as the luminous matter (0:7 : 0:86 : 1), (2) with a more prolate shape (0:5 : 0:66 : 1), and (3) with a more oblate shape (0:7 : 0:93 : 1). Self-consistent solutions by means of the standard orbital superposition technique introduced by Schwarzschild were found in each of the three cases. The equilibrium velocity distribution is reproduced by a Lorentzian function better than by a Gaussian. Chaotic orbits were found to be important in all of the models, and their presence was shown to imply a possible slow evolution of the shapes of the halos. Our results demonstrate for the first time that triaxial dark matter halos can coexist with triaxial galaxies.