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We report the detection of HCO+(J=4-3) emission in the Cloverleaf Quasar atz=2.56, using the IRAM Plateau de Bure Interferometer. HCO+ emission is a starformation indicator similar to HCN, tracing dense molecular hydrogen gas (n(H2)~= 10^5 cm^-3) within star-forming molecular clouds. We derive alensing-corrected HCO+(J=4-3) line luminosity of L'(HCO+(4-3)) = (1.6+/-0.3) x10^9 (mu_L/11)^-1 K km/s pc^2, which corresponds to only 48% of the HCO+(J=1=0)luminosity, and <~4% of the CO(J=3-2) luminosity. The HCO+ excitation thus isclearly subthermal in the J=4-3 transition. Modeling of the HCO+ line radiativetransfer suggests that the HCO+ emission emerges from a region with physicalproperties comparable to that exhibiting the CO line emission, but 2x highergas density. This suggests that both HCO+ and CO lines trace the warm, densemolecular gas where star formation actively takes place. The HCO+ lines haveonly ~2/3 the width of the CO lines, which may suggest that the densest gas ismore spatially concentrated. In contrast to the z=3.91 quasar APM08279+5255,the dense gas excitation in the Cloverleaf is consistent with being purelycollisional, rather than being enhanced by radiative processes. Thus, thephysical properties of the dense gas component in the Cloverleaf are consistentwith those in the nuclei of nearby starburst galaxies. This suggests that theconditions in the dense, star-forming gas in active galactic nucleus-starburstsystems at early cosmic times like the Cloverleaf are primarily affected by thestarburst itself, rather than the central active black hole.

Dense Molecular Gas Excitation at High Redshift: Detection of HCO+(J=4-3) Emission in the Cloverleaf Quasar