  (Abridged) The luminous z=0.286 quasar HE0450-2958 is interacting with a
companion galaxy at 6.5 kpc distance and the whole system is a ULIRG. A so far
undetected host galaxy triggered the hypothesis of a mostly "naked" black hole
(BH) ejected from the companion by three-body interaction. We present new
HST/NICMOS 1.6micron imaging data at 0.1" resolution and VLT/VISIR 11.3micron
images at 0.35" resolution that for the first time resolve the system in the
near- and mid-infrared. We combine these with existing optical HST and CO maps.
  (i) At 1.6micron we find an extension N-E of the quasar nucleus that is
likely a part of the host galaxy, though not its main body. If true, this
places HE0450-2958 directly onto the M_BH-M_bulge-relation for nearby galaxies.
(ii) HE0450-2958 is consistent with lying at the high-luminosity end of
Narrow-Line Seyfert 1 Galaxies, and more exotic explanations like a "naked
quasar" are unlikely. (iii) All 11.3micron radiation in the system is emitted
by the quasar nucleus, which is radiating at super-Eddington rate,
L/L_Edd=6.2+3.8-1.8, or 12 M_sun/yr. (iv) The companion galaxy is covered in
optically thick dust and is not a collisional ring galaxy. It emits in the far
infrared at ULIRG strength, powered by Arp220-like star formation (strong
starburst-like). An M82-like SED is ruled out. (v) With its black hole
accretion rate HE0450-2958 produces not enough new stars to maintain its
position on the M_BH-M_bulge-relation, and star formation and black hole
accretion are spatially disjoint; the bulge has to grow by redistribution of
preexisting stars. (vi) Systems similar to HE0450-2958 with spatially disjoint
ULIRG-strength star formation and quasar activity are rare. At z<0.43 we only
find <4% (3/77) candidates for a similar configuration.
