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

Abstract

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.