Resolving the Host Galaxy of the Nearby QSO I Zw 1 with Subarcsecond Multitransition Molecular Line Observations

We present the first subkiloparsec (~0.″7 ≈ 850 pc) resolution 12CO(1-0) molecular line observations of the interstellar medium (ISM) in the host galaxy of the QSO I Zw 1. The observations were obtained with the BIMA millimeter interferometer in its compact A configuration. The BIMA data are complemented by new observations of the 12CO(2-1) and 13CO(1-0) lines with the IRAM Plateau de Bure Interferometer (PdBI) at 0.″9 and 2.″1 resolution, respectively. These measurements, which are part of a multiwavelength study of the host galaxy of I Zw 1, are aimed at comparing the ISM properties of a QSO host with those of nearby galaxies, as well as to obtain constraints on galaxy formation/evolution models. Our images of the 12CO(1-0) line emission show a ringlike structure in the circumnuclear molecular gas distribution with an inner radius of about 1.2 kpc. The presence of such a molecular gas ring was predicted from earlier lower angular resolution PdBI 12CO(1-0) observations. A comparison of the BIMA data with IRAM PdBI 12CO(2-1) observations shows variations in the excitation conditions of the molecular gas in the innermost 1.″5 comprising the nuclear region of I Zw 1. The observed properties of the molecular cloud complexes in the disk of the host galaxy suggest that they may be the sites of massive circumnuclear star formation and show no indications of excitation by the active galactic nucleus (AGN). This all indicates that the molecular gas in a QSO host galaxy has properties similar to the gas observed in nearby low-luminosity AGNs.