A new probe of dense gas at high redshift:: Detection of HCO+ (5-4) line emission in APM 08279+5255

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) x 10(10) K km s(-1) pc(2). 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 p 5 rotational levels are predominantly excited by infrared fluorescent radiation.