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 H<SUB>2</SUB>conversion factor α<SUB>CO</SUB>= 0.8 M<SUB>☉</SUB>K<SUP>-1</SUP>(km s)<SUP>-1</SUP>pc<SUP>2</SUP>, we infer a molecular gas mass of M(H<SUB>2</SUB>) = (3.2±0.2)×10<SUP>10</SUP>M<SUB>☉</SUB>. Assuming that the observed CO velocity gradient is due to an inclined rotating disk, we derive a dynamical mass of M<SUB>dyn</SUB>sin<SUP>2</SUP>(i)=(2.4 ± 0.5)×10<SUP>10</SUP>M<SUB>☉</SUB>, 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<SUB>BH</SUB>= (1.8 ± 0.5)×10<SUP>9</SUP>M<SUB>☉</SUB>. We find a molecular gas fraction of μ = M(H<SUB>2</SUB>)/M<SUP>*</SUP>∼ 4.4, where M<SUP>∗</SUP>≈ M<SUB>dyn</SUB>- M(H<SUB>2</SUB>) - M(BH). We derive a ratio ν<SUB>rot</SUB>/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. <P />The reduced data cubes and images are only available at the CDS via anonymous ftp to <A href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>(ftp://130.79.128.5) or via <A href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A39">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/619/A39
The dense molecular gas in the z ∼ 6 QSO SDSS J231038.88+185519.7 resolved by ALMA
Bischetti, M.;CRISTIANI, Stefano;
2018-01-01
Abstract
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 H2conversion factor αCO= 0.8 M☉K-1(km s)-1pc2, we infer a molecular gas mass of M(H2) = (3.2±0.2)×1010M☉. Assuming that the observed CO velocity gradient is due to an inclined rotating disk, we derive a dynamical mass of Mdynsin2(i)=(2.4 ± 0.5)×1010M☉, 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: MBH= (1.8 ± 0.5)×109M☉. We find a molecular gas fraction of μ = M(H2)/M*∼ 4.4, where M∗≈ Mdyn- M(H2) - M(BH). We derive a ratio νrot/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/A39Pubblicazioni consigliate
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