A minor merger scenario for the ultraluminous X-ray source ESO 243-49
  HLX-1
  The point-like X-ray source HLX-1 is the brightest known ultraluminous X-ray
source and likely the strongest intermediate-mass black hole candidate. HLX-1
is hosted in the S0 galaxy ESO 243-49, but offset with respect to the nucleus,
and its optical counterpart was identified with a massive star cluster. In this
paper, we study, through N-body/smoothed particle hydrodynamics simulations,
the scenario where ESO 243-49 is undergoing (or just underwent) a minor merger
with a gas-rich low-mass late-type galaxy. The simulations suggest that the
observed star formation rate (SFR) in ESO 243-49 is a consequence of the
interaction and that the companion galaxy already underwent the second
pericentre passage. We propose that the counterpart of HLX-1 coincides with the
nucleus (and possibly with the nuclear star cluster) of the secondary galaxy.
We estimate that, if the minor merger scenario is correct, the number density
of X-ray sources similar to HLX-1 is ~10^-6 Mpc^-3.
