Kavli Affiliate: Rainer Spurzem
| First 5 Authors: Manuel Arca Sedda, Albrecht W. H. Kamlah, Rainer Spurzem, Francesco Paolo Rizzuto, Mirek Giersz
| Summary:
The processes that govern the formation of intermediate-mass black holes
(IMBHs) in dense stellar clusters are still unclear. Here, we discuss the role
of stellar mergers, star-BH interactions and accretion, as well as BH binary
(BBH) mergers in seeding and growing IMBHs in the textsc{Dragon-II} simulation
database, a suite of 19 direct $N$-body models representing dense clusters with
up to $10^6$ stars. textsc{Dragon-II} IMBHs have typical masses of $m_{rm
IMBH} = (100-380)$ M$_odot$ and relatively large spins $chi_{rm IMBH} >
0.6$. We find a link between the IMBH formation mechanism and the cluster
structure. In clusters denser than $3times 10^5$ M$_odot$ pc$^{-3}$, the
collapse of massive star collision products represents the dominant IMBH
formation process, leading to the formation of heavy IMBHs ($m_{rm IMBH} >
200$ M$_odot$), possibly slowly rotating, that form over times $<5$ Myr and
grow further via stellar accretion and mergers in just $<30$ Myr. BBH mergers
are the dominant IMBH formation channel in less dense clusters, for which we
find that the looser the cluster, the longer the formation time ($10-300$ Myr)
and the larger the IMBH mass, although remaining within $200$ M$_odot$. Strong
dynamical scatterings and relativistic recoil efficiently eject all IMBHs in
textsc{Dragon-II} clusters, suggesting that IMBHs in this type of cluster are
unlikely to grow beyond a few $10^2$ M$_odot$.
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