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

¹ INAF – Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, Trieste, Italy
e-mail: chiara.feruglio@inaf.it
² Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge, CB3 0HE UK
³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge, CB3 0HA UK
⁴ Dipartimento di Fisica, Universitá di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone, Italy
⁶ Instituto de Astronomia, Geofísica e Ciências Atmosféricas – Universidade de São Paulo (IAG-USP), Rua do Matão 1226, São Paulo, 05508-090 Brasil
⁷ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁸ Dipartimento di Fisica e Astronomia, Universitá di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
⁹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 2, 50125 Firenze, Italy
¹⁰ Centro Fermi, Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 1, 00184 Roma, Italy

Received: 5 April 2018
Accepted: 7 August 2018

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 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.