MOLECULAR GAS AND THE HOST-GALAXY SYSTEM OF THEz∼ 0.3 QSO PG 1700+518

Abstract

The detection of CO(1→0) emission in the massive (i.e., MH ~ –26.13 mag), z ~ 0.3 host-galaxy system of the broad absorption line quasi-stellar object (QSO) PG1700+518 is reported. The host system has a CO luminosity of L'_CO ~ 1.4 × 10^(10) K km s^(–1) pc^2, and thus a star-forming molecular gas mass of M(H_2) ~ 6 × 10^(10) M_☉ (adopting an α = 4 M_☉[K km s^(–1) pc^2]^(–1)), making it one of the most molecular gas-rich Palomar-Green QSO hosts observed to date. New Hubble Space Telescope WFPC2 direct and NICMOS coronagraphic images show the highest resolution view yet of the host and companion. The new NICMOS image reveals the underlying, apparently tidally disrupted structure seen previously from high-resolution ground-based optical imaging. Light from the host galaxy is overwhelmed by the central point source in the WFPC2 images. The companion galaxy is well resolved in both data sets, and the WFPC2 provides for the first time a clear picture of the optically visible ring structure. The CO redshift is within the range of redshifts derived from optical QSO emission lines, thus the observed CO is associated with the QSO host. However, it cannot be ruled out that the companion has at least ~10^(10) M_☉ of molecular gas. Finally, if the far-infrared luminosity, which is 1/5 of the bolometric luminosity, is the luminosity of the starburst population, the star formation rate is estimated to be ~210 M_☉ yr^(–1). There is thus sufficient molecular gas in the QSO host galaxy to fuel both star formation and QSO activity for another ~10^8 yr. We speculate that we may be witnessing the fueling event in progress that resulted from a collision between the QSO host and the companion galaxy, and that there is an accompanying expulsion of material along our line of sight in the form of broad absorption line gas.