2008A&A...491..173A

Using the IRAM 30m 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 10^3.5-10^4.0cm^–3, and kinetic temperatures between 50K and 200K. The H₂ mass of (3.6-5.4)x10¹⁰M_☉ implied by the line intensities compares well with our estimate of the dynamical mass within the inner 1.5kpc of the object. Fitting a two-component gray body spectrum, we find a dust mass of 1.2x10⁹M_☉, and cold and hot dust temperatures of 42±5K and 160±25K, respectively. The broad MgII line allows us to estimate the mass of the central black hole as 1.7x10⁹M_☉. 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.