Molecular gas in infrared-excess, optically selected quasars and the connection with infrared-luminous galaxies (vol 121, pg 1893, 2001)

The initial results of a millimeter ( CO) survey of infrared-excess, optica lly selected quasars from the Palomar-Green ( PG) Bright Quasar Survey with redshifts in the range 0.04<z<0.17 are presented. These observations repre sent the first step toward establishing with a complete sample whether or n ot quasi-stellar objects ( QSOs) reside in molecular gas-rich galaxies, as well as determining how the infrared and molecular gas properties of QSOs c ompare with those of ultraluminous infrared galaxies (ULIGs), which are a p ossible evolutionary precursor of QSOs. The sample consists of QSOs having absolute blue magnitudes M(B)less than or similar to -22.0 and infrared exc esses (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 luminosit y of infrared thermal dust emission for all PG QSOs is typically 20%-40%. S ix 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+70, and four additional QSOs are detected for the fit time ( PG 1119+120, 1351+640, 1415+451, and 1440+356). These six d etections, plus two previous detections of CO in I Zw 1 and Mrk 1014, bring the total number of 0.04<z< 0.17 infrared-excess PG QSOs detected in CO to date to eight and provide possible evidence that, in addition to fueling s tar 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 no ndetections have high infrared-to-CO luminosity ratios, L-IR/L'(CO), relati ve to most infrared-luminous galaxies of the same L-IR. The placement of th ese QSOs on the L-IR/L'(CO)-L-IR plane may be due to significant contributi ons 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 bot h. If the observed high values of L-IR/L'(CO) are primarily due to dust hea ting by QSOs, a significant fraction of ULIGs with similar values of L-IR/L '(CO) may also contain buried active galactic nuclei. Alternatively, if hig h L-IR/L'(CO) is due primarily to star formation, then an enhanced star for mation rate may be intimately connected to the QSO phenomenon. A comparison of the infrared and CO luminosities of the eight detected and four undetec ted QSOs with the optical morphologies of their host galaxies shows that th e three QSOs with L-IR and L'(CO) similar to ULIGs appear to reside in morp hologically disturbed galaxies (i.e., ongoing major mergers involving two o r 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 spir al host galaxies, elliptical galaxies, galaxies with an indeterminate class ification, and at least one ongoing major merger.