The host galaxies of optically bright quasi-stellar objects:: Molecular gas in z ≤ 0.1 Palomar-Green quasi-stellar objects

We present results of a CO (1-0) line survey in a complete sample of 12 low-redshift (z less than or equal to 0.1), optically bright Palomar-Green quasi-stellar objects (PG QSOs with M-B less than or equal to -23 mag). Six new CO detections are reported here at levels exceeding I-CO similar or equal to 2 Jy km s(-1). Combined with three previously reported detections, we find that nine of CO 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 greater than or equal to 1.0 x 10(9) M-., with the most massive being 10(10) M-. (PG 0050+124; I Zw 1). In the three sources not yet detected in CO, the upper limits on the gas masses are, similar to10(9) 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.