2006ApJ...645L..13R

We report the detection of HCO⁺ (J=1⟶0) emission toward the Cloverleaf quasar (z=2.56) through observations with the Very Large Array. This is the first detection of ionized molecular gas emission at high redshift (z>2). HCO⁺ emission is a star formation indicator similar to HCN, tracing dense molecular hydrogen gas [n(H₂)≃10⁵/cm3] within star-forming molecular clouds. We derive a lensing-corrected HCO⁺ line luminosity of L'_HCO+=3.5x10⁹ K.km/s.pc². Combining our new results with CO and HCN measurements from the literature, we find an HCO⁺/CO luminosity ratio of 0.08 and an HCO⁺/HCN luminosity ratio of 0.8. These ratios fall within the scatter of the same relationships found for low-z star-forming galaxies. However, an HCO⁺/HCN luminosity ratio close to unity would not be expected for the Cloverleaf if the recently suggested relation between this ratio and the far-infrared luminosity were to hold. We conclude that a ratio between HCO⁺ and HCN luminosity close to 1 is likely due to the fact that the emission from both lines is optically thick and thermalized and emerges from dense regions of similar volume. The CO, HCN, and HCO⁺ luminosities suggest that the Cloverleaf is a composite active galactic nucleus-starburst system, in agreement with the previous finding that about 20% of the total infrared luminosity in this system results from dust heated by star formation rather than heating by the active nucleus. We conclude that HCO⁺ is potentially a good tracer for dense molecular gas at high redshift.