Dense Molecular Gas and the Role of Star Formation in the Host Galaxies of Quasi-Stellar Objects
Abstract
New millimeter-wave CO and HCN observations of the host galaxies of infrared-excess Palomar Green quasi-stellar objects (PG QSOs) previously detected in CO are presented. These observations are designed to assess the validity of using the infrared luminosity to estimate star formation rates of luminous AGN by determining the relative significance of dust-heating by young, massive stars and active galactic nuclei (AGN) in QSO hosts and IRAS galaxies with warm, AGN-like infrared colors. The HCN data show the PG QSO host IZw1 and most of the warm IRAS galaxies to have high LIR / L'HCN (>1600) relative to the cool IRAS galaxy population for which the median LIR / L'HCN ~ 890(+440,-470). If the assumption is made that the infrared emission from cool IRAS galaxies is reprocessed light from embedded star-forming regions, then high values of LIR / L'HCN are likely the result of dust heating by the AGN. Further, if the median ratio of L'HCN / L'CO ~ 0.06 observed for Seyfert galaxies and IZw1 is applied to the PG QSOs not detected in HCN, then the derived LIR / L'HCN correspond to a stellar contribution to the production of LIR of ~ 7-39%, and star formation rates ~ 2-37 Msun/yr are derived for the QSO hosts. Alternatively, if the far-infrared is adopted as the star formation component of the total infrared in cool galaxies, the stellar contributions in QSO hosts to their LFIR are up to 35% higher than the percentages derived for LIR. This raises the possibility that the LFIR in several of the PG QSO hosts, including IZw1, could be due entirely to dust heated by young, massive stars. Finally, there is no evidence that the global HCN emission is enhanced relative to CO in galaxies hosting luminous AGN.
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