Molecular gas in low-redshift Palomar-Green quasi-stellar objects

Millimeter-wave (CO and HCN) observations of quasi-stellar objects (QSOs) are potentially a powerful way of studying the properties of both QSOs and their host galaxies. This paper summarizes the state of molecular gas observations of PG QSOs by focussing primarily on a sample of z ⩽ 0.17 Palomar-Green (PG) QSOs with infrared (IR) excesses. Approximately 70% of the PG QSOs observed thus far in CO have detectable amounts of molecular gas, with MH2∼1-36×109M⊙. Optical images of the QSO host galaxies show them to be mixture of spiral, elliptical (i.e., featureless), and merging galaxies. Two QSOs observed at optical wavelengths to reside in featureless, elliptical-like host galaxies are also rich in molecular gas, thus illustrating the usefulness of CO observations in differentiating between old, gas-poor elliptical galaxies and advanced mergers. The CO velocity dispersions of the QSOs are calculated to be ΔvFWHM ∼ 280 km s−1, consistent with that of ultraluminous IR galaxies and suggestive of massive host galaxies. In addition, the QSO host galaxies are observed to have high IR-to-CO luminosity ratios, LIR/LCO′, relative to IR galaxies with similar luminosities, which is an indication of high star formation efficiencies (i.e., LIR/LCO′∼Lstarburst/MH2) or significant dust heating by the AGN. If the naiive assumption is made that the LIR is due entirely to star formation, then star formation rates are estimated to be SFRIR ∼ 20-420M⊙ year−1. However, analysis of recent HCN observations of several PG QSOs, and of [O II] observations of a large sample of QSOs, have provided evidence that a significant fraction of LIR may emanate from dust heated by the AGN.