Intermediate mass black holes from stellar mergers in young star
  clusters
  Intermediate mass black holes (IMBHs) in the mass range
$10^2-10^5\,\mathrm{M_{\odot}}$ bridge the gap between stellar black holes
(BHs) and supermassive BHs. Here, we investigate the possibility that IMBHs
form in young star clusters via runaway collisions and BH mergers. We analyze
$10^4$ simulations of dense young star clusters, featuring up-to-date stellar
wind models and prescriptions for core collapse and (pulsational) pair
instability. In our simulations, only 9 IMBHs out of 218 form via binary BH
mergers, with a mass $\sim{}100-140$ M$_\odot$. This channel is strongly
suppressed by the low escape velocity of our star clusters. In contrast, IMBHs
with masses up to $\sim{}438$ M$_{\odot}$ efficiently form via runaway stellar
collisions, especially at low metallicity. Up to $\sim{}0.2$~% of all the
simulated BHs are IMBHs, depending on progenitor's metallicity. The runaway
formation channel is strongly suppressed in metal-rich ($Z=0.02$) star
clusters, because of stellar winds. IMBHs are extremely efficient in pairing
with other BHs: $\sim{}70$% of them are members of a binary BH at the end of
the simulations. However, we do not find any IMBH-BH merger. More massive star
clusters are more efficient in forming IMBHs: $\sim{}8$% ($\sim{}1$%) of the
simulated clusters with initial mass $10^4-3\times{}10^4$ M$_\odot$
($10^3-5\times{}10^3$ M$_\odot$) host at least one IMBH.
