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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 2006
- DOI:
- 10.1086/505908
- arXiv:
- arXiv:astro-ph/0605437
- Bibcode:
- 2006ApJ...645L..13R
- Keywords:
-
- Cosmology: Observations;
- Galaxies: Active;
- Galaxies: Formation;
- Galaxies: High-Redshift;
- Galaxies: Starburst;
- Radio Lines: Galaxies;
- Astrophysics
- E-Print:
- 5 pages, 3 figures, ApJL, in press (accepted May 17, 2006)