2003ApJ...585L.105S

We present results of a CO (1-0) line survey in a complete sample of 12 low-redshift (z≤0.1), optically bright Palomar-Green quasi-stellar objects (PG QSOs with M_B≤-23 mag). Six new CO detections are reported here at levels exceeding I_CO≃2 Jy km/s. Combined with three previously reported detections, we find that nine of the 12 QSOs have abundant, dense interstellar medium (ISM) characteristic of late-type galaxies. In all nine of the detected QSOs, the derived molecular gas masses are M_H2≥1.0x10⁹ M_☉, with the most massive being 10¹⁰ M_☉(PG 0050+124; I Zw 1). In the three sources not yet detected in CO, the upper limits on the gas masses are ∼10⁹ M_☉, and thus we cannot rule out abundant ISMs even in these objects. Since our sample was chosen entirely on the basis of low redshift and optical luminosity (and not selected for strong infrared emission), we conclude that the majority of luminous, low-redshift QSOs have gas-rich host galaxies and therefore cannot be normal elliptical galaxies.