  We investigate the dynamical evolution of the Orion Nebula Cluster (ONC) by
means of direct N-body integrations. A large fraction of residual gas was
probably expelled when the ONC formed, so we assume that the ONC was much more
compact when it formed compared to its current size, in agreement with the
embedded cluster radius-mass relation from Marks & Kroupa (2012). Hence, we
assume that few-body relaxation played an important role during the initial
phase of evolution of the ONC. In particular, three body interactions among OB
stars likely led to their ejection from the cluster and, at the same time, to
the formation of a massive object via runaway physical stellar collisions. The
resulting depletion of the high mass end of the stellar mass function in the
cluster is one of the important points where our models fit the observational
data. We speculate that the runaway-mass star may have collapsed directly into
a massive black hole (Mbh > 100Msun). Such a dark object could explain the
large velocity dispersion of the four Trapezium stars observed in the ONC core.
We further show that the putative massive black hole is likely to be a member
of a binary system with appr. 70 per cent probability. In such a case, it could
be detected either due to short periods of enhanced accretion of stellar winds
from the secondary star during pericentre passages, or through a measurement of
the motion of the secondary whose velocity would exceed 10 km/s along the whole
orbit.

Astrophysics of Galaxies