The dense molecular gas in the z ∼ 6 QSO SDSS J231038.88+185519.7 resolved by ALMA

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 ± 0.5 kpc, and of 1.4 ± 0.2 kpc for the 91.5 GHz dust continuum. By assuming that CO(6-5) is thermalized, and by adopting a CO to H₂ conversion factor α_CO = 0.8 M_⊙K^-1 (km s)^-1 pc², we infer a molecular gas mass of M(H₂) = (3.2±0.2)×10¹⁰ M_⊙. 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)×10¹⁰ M_⊙, 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)×10⁹ M_⊙. 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.

The reduced data cubes and images are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A39