Dense Molecular Gas at High Redshift: First Detection of Emission from HCO+

Using the Very Large Array (VLA), we have detected the HCO⁺(1-0) emission line towards the Cloverleaf quasar (z = 2.56; Riechers et al. 2006). 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 within star-forming molecular clouds. We find a HCO⁺/CO luminosity ratio of 0.08 and a HCO⁺/HCN luminosity ratio of 0.8 for the Cloverleaf. These ratios fall within the scatter of the same relationships found for low-z star-forming galaxies. However, a 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 (Graciá-Carpio et al. 2006) 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 volumes. We conclude that HCO⁺ is potentially a good tracer for dense molecular gas at high redshift.