INTERSTELLAR-MEDIUM AND STAR-FORMATION IN THE NEARBY QSO-I ZW-1

We report the first detection of the millimeter (CO)-C-13(1-0) and the near-infrared H-2 upsilon = 1-0 S(1) line from the QSO I Zw 1. We also present upper limits on the 1.6435 mum [Fe II] and 1.9615 mum [Si VI] line as well as high-resolution near-infrared continuum data on its nucleus. We interpret the results in light of previously published (CO)-C-12 data on this source and common properties of the far-infrared and molecular line emission of external, luminous infrared galaxies. We propose a two-component model for the molecular line emission in which a core and disk component have similar contributions. In this model the nucleus is dominated by emission from warm, (mostly) optically thick molecular gas, whereas the line emission in the disk is mostly due to cold, probably subthermally excited molecular gas. We also confirm the need for a two-component model for I Zw 1 in which about 2/3 of the far-infrared luminosity originates in the disk and 1/3 originates in the nucleus. We estimate star-forming rate and efficiency for the disk and the nucleus of the I Zw 1 host galaxy and find that the values for both are comparable to those of luminous IRAS galaxies. Over the whole disk the star formation efficiency is close to the maximum value of almost-equal-to 30 L./M. found in Galactic star-forming regions like M17 or W51. Analysis of the nuclear near-infrared colors suggests a mixture of a QSO nucleus and an extincted stellar component contributing about 10%-20% of the flux density at 2.2 mum. This results in an estimate of the molecular bulge size of the order of 1'' to 2'' (1.2-2.4 kpc). However, the presence of the QSO nucleus in I Zw 1 is clearly revealed only by the optical spectrum and the large X-ray luminosity. If our line of sight to the nucleus would be blocked by obscuring material, I Zw 1 would be seen as a luminous IRAS galaxy.