  The paper presents an analysis of the nearby QSO host I Zw 1 based on new
J-band imaging data, obtained with ISAAC at the Very Large Telescope (VLT) of
the European Southern Observatory (ESO). As one of the best-studied QSOs with
exceptional properties lying between those of QSOs, narrow-line Seyfert 1s, and
ultraluminous infrared galaxies, I Zw 1 is a prime candidate for a detailed
case study of a QSO host. With its high angular resolution and sensitivity, the
new J-band image provides strong evidence for an interaction between I Zw 1 and
the western companion galaxy. We present a procedure for subtracting the QSO
nucleus from the I Zw 1 image to obtain an undisturbed view on the underlying
host galaxy. Based on the derived surface brightness profile and the gas
rotation curve, the host is classified as a high-surface-brightness Freeman
type I galaxy with a central disk surface brightness significantly larger than
that of normal spirals. The surface-brightness profile is decomposed into a
Plummer bulge and a Kuzmin disk with similar luminosities and a bulge-to-disk
scale length ratio of 0.29, comparable to that of nearby spiral hosts with
active galactic nuclei (AGN). Different models for the decomposition of the gas
rotation curve are discussed. The resulting J-band mass-to-light ratio (M/L) of
(0.7+/-0.4) M_sun/L_sun for the disk component suggests a mean solar-type
stellar population with a tendency towards young stars. The bulge exhibits a
lower M/L of 0.4+/-0.2 M_sun/L_sun, which supports previous findings of
enhanced starburst activity in the nuclear and circumnuclear region.
