Intermediate-mass black holes in the era of gravitational-wave astronomy

forthcoming

Intermediate-mass black holes (IMBHs), with masses ranging from 10^2 to 10^5 solar masses, are crucial for understanding the growth of supermassive black holes across cosmic ages, but their formation mechanisms remain an open question, despite being in the spotlight for several years. The recent detection of gravitational-wave (GW) events in the IMBH regime demonstrates the potential of GW astronomy to discover IMBHs, and underscores the need for new models to interpret this emerging population. IMblack aims to develop the theoretical framework to probe IMBH formation by exploiting both electromagnetic and GW data. My models will address i) whether, when, and where very massive stars collapse to IMBHs, ii) whether, when, and where runaway stellar collisions in clusters lead to IMBH formation, and iii) the properties of IMBHs born from hierarchical mergers of stellar-sized black holes. This will be key to reconstructing the IMBH mass function, predicting their formation efficiency, and estimating their merger rate density across cosmic time. To achieve these goals, I will implement new methods across various astrophysical domains, including stellar evolution, GW population analysis, star cluster formation, and dynamics. Specifically, I will interface an ambitious suite of stellar collision simulations with advanced stellar evolution calculations, direct N-body models of star cluster dynamics, a population-synthesis semi-analytic approach to explore the multi-dimensional parameter space, and a new framework for the hierarchical assembly of star clusters. I will use GW and electromagnetic data to set constraints on the models. The results of IMblack will pave the way for interpreting GW data from LIGO-Virgo-KAGRA and next-generation interferometers, such as the Cosmic Explorer, Einstein Telescope, and Laser Interferometer Space Antenna.