Dense Molecular Gas and the Role of Star Formation in the Host Galaxies of Quasi-Stellar Ob.pdf

a r X i v : a s t r o - p h / 0 6 0 8 4 5 3 v 1 2 1 A u g 2 0 0 6 Dense Molecular Gas and the Role of Star Formation in the Host Galaxies of Quasi-Stellar Objects A. S. Evans1, P. M. Solomon1, L. J. Tacconi2, T. Vavilkin1, D. Downes3 ABSTRACT New millimeter-wave CO and HCN observations of the host galaxies of infrared-excess Palomar Green quasi-stellar objects (PG QSOs) previously de- tected in CO are presented. These observations are designed to assess the valid- ity 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 analysis of these data is based, in part, on evidence that HCN traces high density ( 104 cm?3) molecular gas, and that the starburst-to-HCN luminosity ratio, Lstarburst/L ′ HCN, of IRAS-detected galaxies is constant. The new CO data provide a confirmation of prior claims that PG QSO hosts have high infrared-to-CO luminosity ratios, LIR/L ′ CO, relative to IRAS galaxies of comparable LIR. Such high LIR/L ′ CO ratios may be due to significant heating of dust by the QSO, or to an increased star formation efficiency in QSO hosts relative to the bulk of the luminous IRAS galaxy population. The HCN data show a similar trend, with the PG QSO host IZw1 and most of the warm IRAS galaxies having high LIR/L ′ HCN( 1600) relative to the cool IRAS galaxy population for which the median 〈LIR/L ′ HCN〉cool ～ 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 product