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, optical ly selected quasars from the Palomar-Green (PG) Bright Quasar Survey with r edshifts 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 w ell as determining how the infrared and molecular gas properties of QSOs co mpare with those of ultraluminous infrared galaxies (ULIGs), which are a po ssible evolutionary precursor of QSOs. The sample consists of QSOs having a bsolute blue magnitudes M-B less than or similar to -22.0 and infrared exce sses (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 li ne; 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 th e first time (PG 1119+120, 1351+640, 1415+451, and 1440+356). These six det ections, plus two previous detections of CO in I Zw1 and Mrk 1014, bring th e total number of 0.04 <z <0.17 infrared-excess PG QSOs detected in CO to d ate to eight and provide possible evidence that, in addition to fueling sta r formation, molecular gas may also serve as a primary source of fuel for Q SO activity. Both the eight QSOs detected in CO and the four QSOs with nond etections 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 thes e QSOs on the L-IR/L'(CO)-L-IR plane may be due to significant contribution s 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 ma ss) 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 heatin g by QSOs, a significant fraction of ULIGs with similar values of L-IR/L'(C O) may also contain buried active galactic nuclei. Alternatively, if high L -IR/L'(CO) is due primarily to star formation, then an enhanced star format ion 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 t hree QSOs with L-IR and L'(CO) similar to ULIGs appear to reside in morphol ogically disturbed galaxies (i.e., ongoing major mergers involving two or m ore gas-rich disk galaxies), whereas the host galaxies of the remaining eig ht QSOs with lower L-IR and L'(CO) appear to be a mixture of barred spiral host galaxies, elliptical galaxies, galaxies with an indeterminate classifi cation, and at least one ongoing major merger.