Jan 13, 2021

Speaker: Lucio Mayer (University of Zurich)

Title: The pathway to massive black hole mergers in the landscape of galaxy formation

Abstract: Since 2016, with the first LIGO/Virgo detection of gravitational waves from merging stellar mass black holes, the new exciting era of gravitational wave astrophysics has begun. Over the next decade the scientific community will prepare to exploit and interpret the data coming from the first gravitational wave detector in space, the Laser Inteferometer Space Antenna (LISA). This will target mergers from supermassive black holes hosted at the center of galaxies, which could be "heard" as far as at z ~ 20, providing a new window on the history of structure formation in the Universe. I will talk about the latest developments in simulating the evolution of LISA massive black hole binaries in both galaxy-scale and sub-pc resolution simulations of galactic nuclei. A novel finding is that, at many different scales, the orbital decay of BH pairs in gaseous environments occurs primarily due to global torques rather than dynamical friction, and that these torques have a significant degree of stochasticity. they are either due to self-gravitating structures ,such as bars and spirals, at galactic scales, or, at smaller scales, resemble planetary migration, but with the added complexity of a clumpy interstellar medium. As a result, the simple predictions made so far with semi-analytical models for LISA event rates are completely insufficient and new approaches are needed. Another important finding is that the dark matter density profile in dwarf galaxies strongly affects the process binary formation, establishing a new intriguing link between dark matter scenarios and LISA detections. Finally, the nature of orbital decay determines the initial conditions of the binary when it enters the gravitational radiation in-spiral phase. The latter is still influenced by the concurrent presence of gas exerting some residual drag as gravitational wave emission begins, which has been largely neglected so far. A new post-newtonian approach that we have developed takes the latter perturbations into account, modifying the standard gravitational wave radiation timescale formula by Peters (1964). These perturbations would show up as phase-shifts of in-spiral wave-forms detectable with LISA, but also as a modulation of event rates.