A Molecular Gas Study of the Initial and Final Stages of the Ultraluminous Infrared Galaxy Phenomenon

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AST-0080881EVANSUltraluminous infrared galaxies (ULIGs) are observed to be closely interacting/ merging galaxies. Both circumnuclear star formation and active galactic nuclear (AGN) activity in ULIGs are believed to be fueled by molecular gas, and the strong ultraviolet-to-optical radiation generated by both processes is absorbed by dust and re-emitted at infrared wavelengths. Optically bright QSOs may be an evolutionary end product of the ULIG phase. The appearance of optically bright QSOs may be the result after outflows and supernovae have cleared much of the dust responsible for obscuring the QSO nucleus.Two projects will be carried out that will provide potential clues to the early and final stages of ULIG phenomenon. The first is a study of the molecular gas properties of ULIG progenitors. Specifically, an observational survey of the Carbon monoxide (CO) (1 to 0 transition) line radiation of ULIGs, whose nuclei are still widely separated, is being undertaken in order to determine the relative molecular gas content of the progenitor galaxies. The CO data are being compared with ground-based and space-based optical to near-infrared images of these ULIGs to verify the location of the molecular gas relative to the stellar nuclei. Preliminary analysis of five ULIGs with nuclear separations of about 5 parsecs shows that ULIGs with warm, Seyfert-like infrared flux ratios have CO associated with only the AGN, whereas ULIGs with cool, starburst-like flux ratios has CO associated with both nuclei. Such a dichotomy may indicate the manner in which mergers with different primary power sources become ultraluminous early in the merger process.The second project is a search for molecular gas remnants of an early ULIG phase in a sample of QSOs. The sample consists of a family of 17 infrared excess QSOs believed to represent the transition stage between the heavily obscured ULIGs and the classic, optical QSOs. The sample has been previously imaged at optical/near infrared wavelengths. These studies have shown evidence of unresolved bright nuclei with underlying spiral arms and tidal tails, as well as luminous young star clusters. The CO observations of these QSOs will provide the statistical sample needed for a direct comparison of their molecular gas properties to those of ULIGs, as well as to the population of gas-rich, high redshift ULIGs and QSOs recently detected in CO.The results of the research will help astronomers understand the differences between galaxies and QSOs.Both of these projects will make use of observational data taken at the Owens Valley Millimeter Array that is operated by Caltech. This project and the Owens Valley Millimeter Array are supported by funds from the Division of Astronomical Sciences. ***