Riechers, Dominik A. and Walter, Fabian and Carilli, Christopher L. and Cox, Pierre and Weiss, Axel and Bertoldi, Frank and Menten, Karl M. (2011) Dense Molecular Gas Excitation at High Redshift: Detection of HCO^+(J = 4 → 3) Emission in the Cloverleaf Quasar. Astrophysical Journal, 726 (1). Art. No. 50. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20110202-104851968
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We report the detection of HCO^+(J = 4 → 3) emission in the Cloverleaf Quasar at z = 2.56, using the IRAM Plateau de Bure Interferometer. HCO^+ emission is a star formation indicator similar toHCN, tracing densemolecular hydrogen gas (n(H_2) ≃ 10^5 cm^(−3)) within star-forming molecular clouds. We derive a lensing-corrected HCO^+(J = 4 → 3) line luminosity of L'_(HCO)^+(4–3) = (1.6 ± 0.3) × 10^9 (μ_L/11)^(−1)K km s^(−1) pc^2, which corresponds to only 48% of the HCO^+(J = 1 → 0) luminosity, and ≲4% of the CO(J = 3→2) luminosity. The HCO^+ excitation thus is clearly subthermal in the J = 4 → 3 transition. Modeling of the HCO^+ line radiative transfer suggests that the HCO^+ emission emerges from a region with physical properties comparable to that exhibiting the CO line emission, but 2 × higher gas density. This suggests that both HCO^+ and CO lines trace the warm, dense molecular gas where star formation actively takes place. The HCO^+ lines have only ~2/3 the width of the CO lines, which may suggest that the densest gas is more spatially concentrated. In contrast to the z = 3.91 quasar APM 08279+5255, the dense gas excitation in the Cloverleaf is consistent with being purely collisional, rather than being enhanced by radiative processes. Thus, the physical properties of the dense gas component in the Cloverleaf are consistent with those in the nuclei of nearby starburst galaxies. This suggests that the conditions in the dense, star-forming gas in active galactic nucleus–starburst systems at early cosmic times like the Cloverleaf are primarily affected by the starburst itself, rather than the central active black hole.
|Additional Information:||© 2011 The American Astronomical Society. Received 2010 October 6; accepted 2010 November 3; published 2010 December 14. We thank the referee for helpful suggestions, and Christian Henkel for the original version of the LVG code. D.A.R. acknowledges support from NASA through Hubble Fellowship grant HST-HF-51235.01 awarded by STScI, operated by AURA for NASA, under contract NAS 5-26555. The IRAM PdBI is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).|
|Subject Keywords:||cosmology: observations – galaxies: active – galaxies: formation – galaxies: high-redshift – galaxies: starburst – radio lines: galaxies|
|Classification Code:||PACS: 98.54.Aj; 98.58.Db; 98.54.Ep; 98.62.Qz; 95.85.Bh; 98.58.Bz|
|Official Citation:||Dominik A. Riechers et al. 2011 ApJ 726 50 doi: 10.1088/0004-637X/726/1/50|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Benjamin Perez|
|Deposited On:||02 Feb 2011 19:37|
|Last Modified:||26 Dec 2012 12:53|
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