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

摘要

We report the detection of HCO+(5-4) emission from the Broad Absorption Line(BAL) quasar APM08279+5255 at z=3.911 based on observations conducted at theIRAM Plateau de Bure interferometer. This represents the first detection ofthis molecular ion at such a high redshift. The inferred line luminosity,uncorrected for lensing, is L'(HCO+)=(3.5+-0.6)x10^10 Kkms^-1pc^2. The HCO+J=5-4 source position coincides within the errors with that reported fromprevious HCN J=5-4 and high-J CO line observations of this quasar. The HCO+line profile central velocity and width are consistent with those derived fromHCN. This result suggests that HCO+(5-4) emission comes roughly from the samecircumnuclear region probed by HCN. However, the HCN(5-4)/HCO+(5-4) intensityratio measured in APM08279+5255 is significantly larger than that predicted bysimple radiative transfer models, which assume collisional excitation and equalmolecular abundances. This could imply that the [HCN]/[HCO^+] abundance ratiois particularly large in this source, or that the J=5 rotational levels arepredominantly excited by IR fluorescent radiation.