Molecular gas in infrared-excess, optically selected and the quasars connection with infrared-luminous galaxies

The initial results of a millimeter (CO) survey of infrared-excess, optically selected quasars from the Palomar-Green (PG) Bright Quasar Survey with redshifts in the range 0.04 <0.17 are presented. These observations represent the first step toward establishing with a complete sample whether or not quasi-stellar objects (QSOs) reside in molecular gas-rich galaxies, as well as determining how the infrared and molecular gas properties of QSOs compare with those of ultraluminous infrared galaxies (ULIGs), which are a possible evolutionary precursor of QSOs. The sample consists of QSOs having absolute blue magnitudes M-B less than or similar to -22.0 and infrared excesses (i.e., infrared to big blue bump luminosity), L-IR(8-1000 mum)L-bbb(0.1-1.0 mum) >0.36, in which the contribution to the bolometric luminosity of infrared thermal dust emission for all PG QSOs is typically 20%-40%. Six out of ten of the QSOs observed are detected in the CO (1-->0) emission line; two detections confirm previous, less sensitive detections of CO (1-->0) in PG 1613+658 and 0838+770, and four additional QSOs are detected for the first time (PG 1119+120, 1351+640, 1415+451, and 1440+356). These six detections, plus two previous detections of CO in I Zw1 and Mrk 1014, bring the total number of 0.04 <0.17 infrared-excess PG QSOs detected in CO to date to eight and provide possible evidence that, in addition to fueling star formation, molecular gas may also serve as a primary source of fuel for QSO activity. Both the eight QSOs detected in CO and the four QSOs with nondetections have high infrared-to-CO luminosity ratios, L-IR/L'(CO), relative to most infrared luminous galaxies of the same L-IR. The placement of these QSOs on the L-IR/L'(CO)-L-IR plane may be due to significant contributions from dust heated by the QSO in their host galaxies, due to dust heated by massive stars formed with high efficiency (i.e., per unit molecular gas mass) relative to most infrared luminous galaxies, or a combination of both. If the observed high values of L-IR/L'(CO) are primarily due to dust heating by QSOs, a significant fraction of ULIGs with similar values of L-IR/L'(CO) may also contain buried active galactic nuclei. Alternatively, if high L-IR/L'(CO) is due primarily to star formation, then an enhanced star formation rate may be intimately connected to the QSO phenomenon. A comparison of the infrared and CO luminosities of the eight detected and four undetected QSOs with the optical morphologies of their host galaxies shows that the three QSOs with L-IR and L'(CO) similar to ULIGs appear to reside in morphologically disturbed galaxies (i.e., ongoing major mergers involving two or more gas-rich disk galaxies), whereas the host galaxies of the remaining eight QSOs with lower L-IR and L'(CO) appear to be a mixture of barred spiral host galaxies, elliptical galaxies, galaxies with an indeterminate classification, and at least one ongoing major merger.