A minor merger scenario for the ultraluminous X-ray source ESO 243-49
  HLX-1 - II. Constraints from photometry
  The point-like X-ray source HLX-1, close to the S0 galaxy ESO 243-49, is the
brightest known ultraluminous X-ray source and one the strongest
intermediate-mass black hole candidates. We argue that the counterpart of HLX-1
may be the nucleus of a satellite galaxy, undergoing minor merger with the S0
galaxy. We investigate this scenario by running a set of N-body/smoothed
particle hydrodynamics simulations of the minor merger between a S0 galaxy and
a gas-rich bulgy satellite galaxy, and by comparing the results with the
available photometric Hubble Space Telescope (HST) data of ESO 243-49 and of
the HLX-1 counterpart. In particular, we derive synthetic surface brightness
profiles for the simulated counterpart of HLX-1 in six HST filters, ranging
from far ultraviolet (FUV) to infrared wavelengths. Such synthetic profiles
include a contribution from the stellar population associated with the
simulated disrupted satellite and a contribution from an irradiated disc model.
These are in reasonable agreement with the observed surface brightness profiles
of the HLX-1 counterpart, provided that the merger is at sufficiently late
stage (>~2.5 Gyr since the first pericentre passage). The main difference
between models and observations is in the FUV band, where the HST image shows a
fuzzy and extended emission. We show that the spectral energy distribution of
the bulge of ESO 243-49 cannot be explained with a single old stellar
population, but requires the existence of a younger stellar component. This is
in good agreement with the star formation history derived from our N-body
simulations, and is a further hint for the minor merger scenario.
