First Detection of HCO + Emission at High Redshift

Abstract

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 cm-3] within star-forming molecular clouds. We derive a lensing-corrected HCO+ line luminosity of L′HCO+ = 3.5 × 109 K km s-1 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. © 2006. The American Astronomical Society. All rights reserved.