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This paper addresses the origin of the far-infrared (FIR) continuum of QSOs,based on the Quasar and ULIRG Evolution Study (QUEST) of nearby QSOs and ULIRGsusing observations with the Spitzer Space Telescope. For 27 Palomar-Green QSOsat z <~ 0.3, we derive luminosities of diagnostic lines ([NeII]12.8um,[NeV]14.3um, [OIV]25.9um) and emission features (PAH7.7um emission which isrelated to star formation), as well as continuum luminosities over a range ofmid- to far-infrared wavelengths between 6 and 60um. We detect star-formationrelated PAH emission in 11/26 QSOs and fine-structure line emission in all ofthem, often in multiple lines. The detection of PAHs in the average spectrum ofsources which lack individual PAH detections provides further evidence for thewidespread presence of PAHs in QSOs. Similar PAH/FIR and [NeII]/FIR ratios arefound in QSOs and in starburst-dominated ULIRGs and lower luminositystarbursts. We conclude that the typical QSO in our sample has at least 30% butlikely most of the far-infrared luminosity (~ 10^(10...12)Lsun) arising fromstar formation, with a tendency for larger star formation contribution at thelargest FIR luminosities. In the QSO sample, we find correlations between mostof the quantities studied including combinations of AGN tracers and starbursttracers. The common scaling of AGN and starburst luminosities (and fluxes) isevidence for a starburst-AGN connection in luminous AGN. Strong correlations offar-infrared continuum and starburst related quantities (PAH, low excitation[NeII]) offer additional support for the starburst origin of far-infraredemission.Comment: 39 pages, 8 figures, accepted for publication in Ap

SpitzerQuasar and ULIRG Evolution Study (QUEST). I. The Origin of the Far‐Infrared Continuum of QSOs