DENSE MOLECULAR GAS EXCITATION IN NUCLEAR STARBURSTS AT HIGH REDSHIFT: HCN, HNC, AND HCO+(J=6→5) EMISSION IN THE z=3.91 QUASAR HOST OF APM 08279+5255

We report the detection of surprisingly strong HCN(J = 6 -> 5), HNC(J = 6 -> 5), and HCO+(J = 6 -> 5) emission in the host galaxy of the z = 3.91 quasar APM 08279+5255 through observations with the Combined Array for Research in Millimeter-wave Astronomy. HCN, HNC, and HCO+ are typically used as star formation indicators, tracing dense molecular hydrogen gas [n(H-2) > 10(5) cm(-3)] within star-forming molecular clouds. However, the strength of their respective line emission in the J = 6 -> 5 transitions in APM 08279+5255 is extremely high, suggesting that they are excited by another mechanism besides collisions in the dense molecular gas phase alone. We derive J = 6 -> 5 line luminosities of L'(HCN) = (4.9 +/- 0.6), L'(HNC) = (2.4 +/- 0.7), and L'(HCO+) = (3.0 +/- 0.6) x 10(10) mu(-1)(L) K km s(-1) pc(2) (where mu(L) is the lensing magnification factor), corresponding to L' ratios of similar to 0.23-0.46 relative to CO(J = 1 -> 0). Such high line ratioswould be unusual even in the respective ground-state (J = 1 -> 0) transitions, and indicate exceptional, collisionally and radiatively driven excitation conditions in the dense, star-forming molecular gas in APM 08279+5255. Through an expansion of our previous modeling of the HCN line excitation in this source, we show that the high rotational line fluxes are caused by substantial infrared pumping at moderate opacities in a similar to 220 K warm gas and dust component. This implies that standard M-dense/L' conversion factors would substantially overpredict the dense molecular gas mass M-dense. We also find a HCN(J = 6 -> 5)/HCN(J = 5 -> 4) L' ratio greater than 1 (1.36 +/- 0.31)-however, our models show that the excitation is likely not super-thermal, but that the high line ratio is due to a rising optical depth between both transitions. These findings are consistent with the picture that the bulk of the gas and dust in this source is situated in a compact, nuclear starburst, where both the highly active galactic nucleus and star formation contribute to the heating.