2018A&A...619A..39F

We present ALMA observations of the CO(6-5) and [CII] emission lines and the sub-millimeter continuum of the z∼6 quasi-stellar object (QSO) SDSS J231038.88+185519.7. Compared to previous studies, we have analyzed a synthetic beam that is ten times smaller in angular size, we have achieved ten times better sensitivity in the CO(6-5) line, and two and half times better sensitivity in the [CII] line, enabling us to resolve the molecular gas emission. We obtain a size of the dense molecular gas of 2.9±0.5kpc, and of 1.4±0.2kpc for the 91.5GHz dust continuum. By assuming that CO(6-5) is thermalized, and by adopting a CO to H₂ conversion factor α_CO=0.8M_☉/K/(km/s).pc², we infer a molecular gas mass of M(H₂)=(3.2±0.2)x10¹⁰M_☉. Assuming that the observed CO velocity gradient is due to an inclined rotating disk, we derive a dynamical mass of M_dynsin²(i)=(2.4±0.5)x10¹⁰M_☉, which is a factor of approximately two smaller than the previously reported estimate based on [CII]. Regarding the central black hole, we provide a new estimate of the black hole mass based on the C IV emission line detected in the VLT/X-shooter spectrum: M_BH=(1.8±0.5)x10⁹M_☉. We find a molecular gas fraction of µ=M(H₂)/M^*∼4.4, where M^*~= M_dyn-M(H₂)-M(BH). We derive a ratio ν_rot/σ~=1-2 suggesting high gas turbulence, outflows/inflows and/or complex kinematics due to a merger event. We estimate a global Toomre parameter Q∼0.2-0.5, indicating likely cloud fragmentation. We compare, at the same angular resolution, the CO(6-5) and [CII] distributions, finding that dense molecular gas is more centrally concentrated with respect to [CII]. We find that the current BH growth rate is similar to that of its host galaxy.