EDP Sciences
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Volume 491, Number 1, November III 2008
Page(s) 173 - 181
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:200810628
Published online 17 September 2008

A&A 491, 173-181 (2008)
DOI: 10.1051/0004-6361:200810628

Properties of the molecular gas in a starbursting QSO at z = 1.83 in the COSMOS field

M. Aravena1, 2, F. Bertoldi1, E. Schinnerer3, A. Weiss2, K. Jahnke3, C. L. Carilli4, D. Frayer5, C. Henkel2, M. Brusa6, K. M. Menten2, M. Salvato7, and V. Smolcic7

1  Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
    e-mail: maraven@astro.uni-bonn.de
2  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
3  Max-Planck-Institut für Astronomie, Königsstuhl 17, 69117 Heidelberg, Germany
4  National Radio Astronomy Observatory, PO Box, Socorro, NM 87801, USA
5  Infrared Processing and Analysis Center, California Institute of Technology 100-22, Pasadena, CA 91125, USA
6  Max-Planck Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
7  California Institute of Technology 105-24, Pasadena, CA 91125, USA

Received 17 July 2008 / Accepted 4 September 2008

Using the IRAM 30 m telescope, we have detected the 12CO J=2-1, 4-3, 5-4, and 6-5 emission lines in the millimeter-bright, blank-field selected AGN COSMOS J100038+020822 at redshift z = 1.8275. The sub-local thermodynamic equilibrium (LTE) excitation of the J=4 level implies that the gas is less excited than that in typical nearby starburst galaxies such as NGC 253, and in the high-redshift quasars studied to date, such as J1148+5251 or BR1202-0725. Large velocity gradient (LVG) modeling of the CO line spectral energy distribution (CO SED; flux density vs. rotational quantum number) yields H2 densities in the range 103.5-104.0 cm-3, and kinetic temperatures between 50 K and 200 K. The H2 mass of (3.6 - 5.4)  $\times$  1010 $M_{\odot}$ implied by the line intensities compares well with our estimate of the dynamical mass within the inner 1.5 kpc of the object. Fitting a two-component gray body spectrum, we find a dust mass of 1.2 $\times$ 109 $M_{\odot}$, and cold and hot dust temperatures of 42$\pm$5 K and 160$\pm$25 K, respectively. The broad MgII line allows us to estimate the mass of the central black hole as 1.7 $\times$ 109 $M_{\odot}$. Although the optical spectrum and multi-wavelength SED matches those of an average QSO, the molecular gas content and dust properties resemble those of known submillimeter galaxies (SMGs). The optical morphology of this source shows tidal tails that suggest a recent interaction or merger. Since it shares properties of both starburst and AGN, this object appears to be in a transition from a strongly starforming submillimeter galaxy to a QSO.

Key words: galaxies: evolution -- galaxies: starburst -- galaxies individual: J100038+020822 -- galaxies: ISM -- galaxies: quasars: emission lines -- galaxies: high-redshift

© ESO 2008