A New Probe of Dense Gas at High Redshift: Detection of HCO + (5-4) Line Emission in APM 08279+5255

Abstract

We report the detection of HCO^{+} (5-4) emission from the broadabsorption line quasar APM 08279+5255 at z=3.911 based on observationsconducted with the IRAM Plateau de Bure Interferometer. This representsthe first detection of this molecular ion at such a high redshift.The inferred line luminosity, uncorrected for lensing, is L^{'}_{HCO+}=(3.5+/-0.6){\times}10^{10}K km s^{-1} pc^{2}. The HCO^{+} J=5-4 source position coincides withinthe errors with that reported from previous HCN J=5-4 and high-JCO line observations of this quasar. The HCO^{+} line profile centralvelocity and width are consistent with those derived from HCN. Thisresult suggests that HCO^{+} (5-4) emission comes roughly from thesame circumnuclear region probed by HCN. However, the HCN (5-4)/HCO^{+}(5-4) intensity ratio measured in APM 08279+5255 is significantlylarger than that predicted by simple radiative transfer models, whichassume collisional excitation and equal molecular abundances. Thiscould imply that the [HCN]/[HCO^{+}] abundance ratio is particularlylarge in this source, or that the J=5 rotational levels are predominantlyexcited by infrared fluorescent radiation.Based on observations carried out with the IRAM Plateau de Bure Interferometer.IRAM is supported by the Institut National des Sciences de l'Universdu CNRS (France), the Max-Planck-Gesellschaft (Germany), and theInstituto Geogr{á}fico Nacional (Spain).