Abstract
LIGO and Virgo have reported the detection of GW190521, from the merger of a
binary black hole (BBH) with a total mass around $150$ M$_ødot$. While current
stellar models limit the mass of any black hole (BH) remnant to about $40 - 50$
M$_ødot$, more massive BHs can be produced dynamically through repeated
mergers in the core of a dense star cluster. The process is limited by the
recoil kick (due to anisotropic emission of gravitational radiation) imparted
to merger remnants, which can escape the parent cluster, thereby terminating
growth. We study the role of the host cluster metallicity and escape speed in
the buildup of massive BHs through repeated mergers. Almost independent of host
metallicity, we find that a BBH of about $150$ M$_ødot$ could be formed
dynamically in any star cluster with escape speed $200$ km s$^-1$, as
found in galactic nuclear star clusters as well as the most massive globular
clusters and super star clusters. We compute the detection probability for
different primary masses ($60$ M$_ødot$) as a function of secondary mass
and find that the detection probability increases with secondary mass and
decreases for larger primary mass and redshift. Future additional detections of
massive BBH mergers will be of fundamental importance for understanding the
growth of massive BHs through dynamics and the formation of intermediate-mass
BHs.
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