Dynamical mergers of star systems over a wide range of scales

Mergers are one of the main channels of formation and evolution of complex systems in the universe. Most of the late elliptical galaxies are believed to be the result of merger between spiral galaxies. The densest star systems ever discovered, the Nuclear Star Clusters (NSCs), hosted in many galactic centers, are thought to be formed through the merger between globular clusters orbitally decayed. Supermassive Black Holes (SMBHs) with masses greater than 10’9 M could be the result of merger between smaller black holes, driven by galactic collisions. The interaction and collision between galaxies occur at scales of the order of thousands of kiloparsecs; mergers of star clusters in the galactic nuclei involve distances of the order of tens of parsecs; while mergers between compact objects deal with scales of few parsecs or even less than one parsec. Even though the spatial scales involved in those examples are very different, the presence of an environment around the system, and its physical properties, is crucial to let the objects to go close enough that their mutual gravity overcomes any other force. At great scales is the intergalactic medium (IGM), composed mainly of gas and dust, that brakes the galaxies and gradually reduces their orbital energy. In the galactic nuclei this role is played by the high density field of stars, that let the orbiting clusters to decay in the center by dynamical friction. Almost the same holds for the shrinking of SMBH binaries, but in that case another mechanism, directly connected with the small scales at which this process occurs, contributes to extract orbital energy: this is the effect of the stochastic gravitational encounters between the binary and the field stars. The present work has the aim to study mergers at the three different scales discussed above, finding the characteristic time range for the completion of the process and the properties of the merger remnant. To do this we have used high precision codes able to compute the mutual force between each star and realize a reliable simulation of what could happen in nature. For the galactic scale we have focused on a well-known system of galaxies: the pair composed by our Galaxy, the Milky Way (MW) and the nearby Andromeda Galaxy, also known as M31. Since we have strong evidences that at the centers of those galaxies are hosted two SMBHs, we have included such compact objects in our galactic models and follow their orbital evolution during the galaxy interaction down to very small scales, until they form a tight binary and finally merge. For middle scales we have chosen to study the dynamics of the merger of two star clusters in a galactic nucleus. This could be possible starting from observations of the spiral galaxy NGC 4654 that reveal two bright clumps near the center. After assuming some reasonable values for their actual distances and velocities, we have studied the timing of the process, the properties of the final cluster that results from the merger, and investigate the possibility to form a NSC. Through these studies we want to provide a wide spectrum analysis of the mergers between star systems and compact objects in order to show similarities and differences and depict a complete picture of the fundamental features of such a process.