  The QSO HE0450-2958 was brought to the front scene by the non-detection of
its host galaxy and strong upper limits on the latter's luminosity. The QSO is
also a powerful infrared emitter, in gravitational interaction with a strongly
distorted UltraLuminous InfraRed companion galaxy. We investigate the
properties of the companion galaxy, through new near- and mid-infrared
observations of the system obtained with NICMOS onboard HST, ISAAC and VISIR on
the ESO VLT. The companion galaxy is found to harbour a point source revealed
only in the infrared, in what appears as a hole or dark patch in the optical
images. Various hypotheses on the nature of this point source are analyzed and
it is found that the only plausible one is that it is a strongly reddened AGN
hidden behind a thick dust cloud. The hypothesis that the QSO supermassive
black hole might have been ejected from the companion galaxy in the course of a
galactic collision involving 3-body black holes interaction is also reviewed,
on the basis of this new insight on a definitely complex system.
