FORMATION OF NUCLEAR STAR CLUSTERS AROUND MASSIVE BLACK HOLES: IMPLICATIONS FOR GRAVITATIONAL WAVE SOURCES

Many galaxies show nucleated central regions called Nuclear Star Clusters (NSCs). The study of NSCs is intriguing because supermassive black holes (SMBHs) obey similar scaling relations even though the host galaxies show little evidence of nucleation (Ferrarese et al. 2006). This suggests that the fueling and growth of NSCs and SMBHs are regulated by the same processes, so that these systems may be grouped into a single class of objects referred to as "central massive objects". The existence of such correlations indicates a link among large galactic spatial scales and the much smaller scale of the nuclear environment. Two possibilities have been raised for NSC formation: in-situ or dissipationless origin. An in-situ origin bases on gas radial inflow into the galactic center and requires efficient dissipation mechanisms to work (e.g., Loose et al. 1982). In this model, a NSC consists mostly of stars that formed locally (Schinnerer et al. 2006). Actually, this in-situ star formation model remains qualitative because of the inherent complexity of the gas dynamics. Alternatively, NSCs could have a dissipationless origin in which massive stellar clusters migrate due to dynamical friction and form a dense nucleus up to the size of observed NSCs. Observations of NSCs in dE galaxies suggest that the majority of these systems could be the result of accumulating mass in form of orbitally decayed globular clusters. Numerical studies show that such model is consistent with the measured sizes and luminosities of nuclei (Bekki et al. 2004; Capuzzo-Dolcetta 1993; Capuzzo-Dolcetta & Miocchi 2008). On the other side, a radial density and velocity profile can be reliably determined only for the Milky Way's NSC, which is close enough (about 8 kpc away) to be resolved into individual stars (Schoedel et al. 2009). The Milky Way's NSC has an estimated mass of 10^7 Mimm 73 (hereafter imm 73 refers to solar units) (Schoedel et al. 2008), and it hosts a massive black hole whose mass, 4x10^6 Mimm 73, is uniquely well determined (Gillessen et al. 2009). A handful of other galaxies are also known to contain both a NSC and an SMBH (Seth et al. 2008; Graham & Spitler 2009), with comparable masses. In models of NSCs, the dynamical influence of an SMBH should therefore be considered, at least in bulges brighter than about 10^9 Limm 73. which are believed to always contain an SMBH (Ferrarese & Ford 2005). Understanding systems like the Milky Way's NSC is crucial for predictions on the event rates for low frequency gravitational wave (GW) detectors. Galactic nuclei are the location of many interesting processes that will be potential gravitational wave sources for LISA-like space-based interferometers (Hughes 2003). These include: (i) the capture of stellar-mass black holes by SMBHs called "extreme mass-ratio inspirals" (EMRIs), and (ii) "intermediate mass-ratio inspirals" (IMRIs) of intermediate-mass black holes (IMBHs) into SMBHs. The research projects presented below aim to understand the formation and evolution of NSCs around SMBHs, the distribution of compact remnants in galactic nuclei, and the dynamical evolution of massive black hole binaries having an intermediate mass ratio.