MOLECULAR GAS AND STAR-FORMATION IN THE HOST GALAXY OF THE QSO-I ZW-1

We have investigated the ISM of the I Zw 1 QSO host galaxy with Platea u de Bure millimeter interferometry and high angular resolution near-i nfrared imaging spectroscopy. We have detected a circumnuclear gas rin g of diameter similar to 1.''5 (1.8 kpc) in its millimetric CO line em ission and have mapped the disk and the spiral arms of the host galaxy in the (CO)-C-12(1-0) line at 115 GHz as well as in the H-(1.65 mu m) and K-(2.2 mu m) bands. Combining our new millimeter and NIR data wit h available estimates of the radio and far-infrared contributions to t he nuclear emission, we find strong evidence for a nuclear starburst r ing. A comparison to other sources with nuclear activity indicates tha t these rings may be a common phenomenon and may contribute a large fr action of the central luminosity. Both the CO rotation curve as well a s the NIR and optical images are consistent with an inclination of 38 degrees +/- 5 degrees. Using this, we obtain a total dynamical mass of (3.9 +/- 1.6) X 10(10) M-. and a cold molecular gas mass of (7.5 +/- 1.5) X 10(9) M-. for the inner 3.9 kpc. With an estimate of the nuclea r stellar contribution to the mass and light from NIR spectroscopy and assuming that the contribution of the H I gas to the overall mass of the inner 3.9 kpc is small, we derive an N-H2/I-CO conversion factor c lose to 2 X 10(20) cm(-2) K-1 km(-1) s found for molecular gas in our Galaxy and many nearby external galaxies. A comparison to broadband sp ectra of spiral galaxies, ellipticals, and the nucleus and disk in NGC 7469 suggests bluer disk colors for I Zw 1 and that star formation in the host galaxy and the western companion of I Zw 1 is enhanced. This is also supported by a starburst analysis using all available data on the northwestern spiral arm. The presence of molecular material withi n the disk and on the arm indicates that, at least in this region 12 k pc from the nucleus, star formation, and not scattered light from the QSO nucleus, is responsible for the blue disk colors.