Paola Vaccaro   (Italy)

mariapaolavaccaro @ gmail.com

The population of binary black holes in the local universe

With the advent of the third-generation interferometers LIGO, Virgo, and KAGRA, it is now possible to observe the gravitational waves emitted by compact binary mergers. A few binary black hole (BBH) candidates like GW190521 and possibly GW190403_051519 and GW190426_190642 stand out among the other detections because they involve the mergers of black holes (BHs) in the pair-instability mass gap, challenging traditional models of stellar evolution and raising questions about their formation. During my PhD, I will investigate the formation of such objects through stellar evolution in high-multiplicity systems. For instance, repeated mergers of stellar-origin BHs (also called hierarchical mergers) in active galactic nuclei (AGNs) are unique among formation channels of BBHs because they are likely associated with electromagnetic counterparts and can efficiently lead to the mass growth of BHs.

In my work, I explore the impact of gas accretion and migration traps on the evolution of BBHs in AGNs. I'm developing a new fast semi-analytic model, which allows us to explore the parameter space while capturing the main physical processes involved. I find that effective exchange of energy and angular momentum between the BBH and the surrounding gas (hereafter, gas hardening) during inspiral greatly enhances the efficiency of hierarchical mergers, leading to the formation of intermediate-mass BHs (up to 10.000 solar masses) and triggering spin alignment. Moreover, my models with efficient gas hardening show both an anti-correlation between BBH mass ratio and effective spin, and a correlation between primary BH mass and effective spin. In contrast, if gas hardening is inefficient, the hierarchical merger chain is already truncated after the first two or three generations. I compare the BBH population in AGNs with other dynamical channels as well as isolated binary evolution.

Supervisor:    Michela Mapelli  (ITA)

*) = Quereinsteiger