  The likely merger process and the properties of the stellar populations in
the I Zw 1 host galaxy are analyzed on the basis of multi-wavelength
observations (with the ISAAC camera at the Very Large Telescope (VLT/UT1) of
the European Southern Observatory (ESO), Chile (Paranal), with the
interferometer of the Berkeley-Illinois-Maryland Association (BIMA), USA (Hat
Creek/California), and with the IRAM Plateau de Bure Interferometer (PdBI),
France) and N-body simulations. The data give a consistent picture of I Zw 1,
with properties between those of ultra-luminous infrared galaxies (ULIRGs) and
QSOs as displayed by transition objects in the evolutionary sequence of active
galaxies.
