Molecular Gas and Star Formation in the Host Galaxy of the QSO I ZW 1

Abstract

We have investigated the ISM of the I Zw 1 QSO host galaxy with Plateau de Bure millimeter interferometry and high angular resolution near-infrared imaging spectroscopy. We have detected a circumnuclear gas ring of diameter ~1.5" (1.8 kpc) in its millimetric CO line emission and have mapped the disk and the spiral arms of the host galaxy in the ¹²CO(1-0) line at 115 GHz as well as in the H- (1.65 μm) and K- (2.2 μm) bands. Combining our new millimeter and NIR data with available estimates of the radio and far-infrared contributions to the nuclear emission, we find strong evidence for a nuclear starburst ring. A comparison to other sources with nuclear activity indicates that these rings may be a common phenomenon and may contribute a large fraction of the central luminosity. Both the CO rotation curve as well as the NIR and optical images are consistent with an inclination of 38° +/- 5°. Using this, we obtain a total dynamical mass of (3.9 +/- 1.6) × 10¹⁰ M_⊙ and a cold molecular gas mass of (7.5 +/- 1.5) × 10⁹ M_⊙ for the inner 3.9 kpc. With an estimate of the nuclear 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 close to 2 × 10²⁰ cm^-2 K^-1 km^-1 s found for molecular gas in our Galaxy and many nearby external galaxies. A comparison to broadband spectra 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 within the disk and on the arm indicates that, at least in this region 12 kpc from the nucleus, star formation, and not scattered light from the QSO nucleus, is responsible for the blue disk colors.