Kavli Affiliate: Daniel E. Holz
| First 5 Authors: Michael Zevin, Daniel E. Holz, , ,
| Summary:
The population of binary black hole mergers identified through gravitational
waves has uncovered unexpected features in the intrinsic properties of black
holes in the Universe. One particularly surprising and exciting result is the
possible existence of black holes in the pair-instability mass gap,
$sim50-120~M_odot$. Dense stellar environments can populate this region of
mass space through hierarchical mergers, with the retention efficiency of black
hole merger products strongly dependent on the escape velocity of the host
environment. We use simple toy models to represent hierarchical merger
scenarios in various dynamical environments. We find that hierarchical mergers
in environments with high escape velocities ($gtrsim300$ km/s) are efficiently
retained, leading to an abundance of high-mass mergers that is potentially
incompatible with the empirical mass spectrum from the current catalog of
binary black hole mergers. Models which efficiently generate hierarchical
mergers must therefore be tuned to avoid over-producing binary black hole
mergers within and above the pair-instability mass gap. If hierarchical
formation is indeed an important feature of dynamical formation channels with
high escape velocities, it must be inhibited by some physical mechanism to
avoid a cluster catastrophe that produces too high a rate of enormous
stellar-mass black hole mergers.
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