First Detection of Hco Emission at High Redshift

We report the detection of HCO(J=1→0) emission towards 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) ≃ 10 5 cm) within star–forming molecular clouds. We derive a lensing–corrected HCO line luminosity of L′HCO+ = 3.5× 10 K kmspc. Combining our new results with CO and HCN measurements from the literature, we find a HCO/CO luminosity ratio of 0.08 and a 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, 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 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. The CO, HCN and HCO luminosities suggest that the Cloverleaf is a composite AGN–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 AGN. We conclude that HCO is potentially a good tracer for dense molecular gas at high redshift. Subject headings: galaxies: active, starburst, formation, high redshift — cosmology: observations — radio lines: galaxies