Dynamical fingerprints of black holes in globular clusters

Abstract

Globular clusters (GCs), compact stellar systems orbiting in and around galaxies, are natural laboratories to study a diverse range of astrophysical processes. The current stellar population of the oldest GCs in our Galaxy is the manifestation of more than 12 billion years of combined stellar, dynamical and hydrodynamical evolution, whose interplay is responsible for enhanced presence of star exotica such as millisecond pulsars, blue stragglers and black hole (BH) binaries. GCs have also been indicated as possible formation sites of intermediate-mass black holes (IMBHs), which might represent the missing link between the wellknown populations of stellar BHs (few tens times the Sun’s mass) and supermassive BHs (more than a million times the Sun’s mass). Despite recent efforts, a clear evidence of their existence is still missing, therefore identifying multiple signatures of their presence has become critical.

In the first part of this thesis, we address two main issues that may affect a possible IMBH detection. The first issue is represented by the systematic uncertainties in classical observational techniques (e.g., integrated-light IFU spectroscopy). In particular, we use state-of-the-art numerical simulations to produce realistic mock observations considering different setups in order to assess under which conditions the presence of an IMBH can be successfully recovered. The second issue is related to the IMBH wandering off-center, which is fundamental to take into account, especially when the presence of IMBHs is constrained through dynamical modeling of stellar kinematics. Guided by the simulation results, we developed a basic yet accurate model that can be used to estimate the average IMBH radial displacement in terms of structural quantities, which can be constrained by the observations.

In the second part of the thesis, we present a new set of cutting-edge direct Nbody simulations, which have been specifically designed to study the dynamical influence of BHs on the long-term evolution of GCs. We combined our numerical simulations with analysis techniques from high-resolution observations of GCs with the aim of identifying key indicators that correlate with the black hole mass fraction. Our results offer novel approaches to indirectly characterise black hole populations in star clusters, which in turn can constrain theories of globular cluster formation and estimates of dynamically-induced gravitational wave merging rates.