2006ApJ...645L..17G

We report the detection of HCO⁺ (5-4) emission from the broad absorption line quasar APM 08279+5255 at z=3.911 based on observations conducted with the IRAM Plateau de Bure Interferometer. This represents the 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)x10¹⁰ K.km/s.pc². The HCO⁺ J=5-4 source position coincides within the errors with that reported from previous 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 from HCN. This result suggests that HCO⁺ (5-4) emission comes roughly from the same circumnuclear region probed by HCN. However, the HCN (5-4)/HCO⁺ (5-4) intensity ratio measured in APM 08279+5255 is significantly larger than that predicted by simple radiative transfer models, which assume collisional excitation and equal molecular abundances. This could imply that the [HCN]/[HCO⁺] abundance ratio is particularly large in this source, or that the J=5 rotational levels are predominantly excited by infrared fluorescent radiation.