Alessia Gualandris - April 6, 2018

Collisionless losscone refilling: the end of the final parsec problem

Mergers of massive black hole binaries (BHBs) are expected to be a key source of
gravitational waves (GWs). However, the expected number and frequency
of such mergers remains highly uncertain. This owes to a
long-standing puzzle known as the final parsec problem. Gravitational
N-body simulations that assume a spherical distribution of stars
surrounding the BHB find that the binary stalls when all stars on
intersecting orbits have been ejected. This is inconsistent with
available observations: every massive galaxy observed to date appears
to host a single supermassive black hole, and only a handful of BHB
candidates exist.  
I will review the final parsec problem and present a new approach to
determine whether collisional repopulation of the binary's losscone in
non-spherical nuclei is a viable mechanism to drive binary coalescence.
I will present results of direct summation N-body simulations of
galaxy mergers, running on multiple GPUs. These allow to measure the binary 
hardening rate and determine a new proxy that depends only on the 
stars' angular momentum.
I will then present collisionless simulations of spherical and
non-spherical isolated models to increase the number of particles
sufficiently that collisional effects become unimportant and
collisionless losscone refilling can be measured.