Molecular outflow and feedback in the obscured quasar XID2028 revealed by ALMA

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
e-mail: marcella.brusa3@unibo.it
² INAF – Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁴ CEA, IRFU, DAp, AIM, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, CNRS, 91191 Gif-sur-Yvette, France
⁵ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁶ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone (RM), Italy
⁷ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁸ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK
⁹ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo, 11, 34143 Trieste, Italy
¹⁰ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching bei München, Germany
¹² Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
¹³ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁴ Dipartimento di Fisica e Astronomia, Università di Firenze, via G. Sansone 1, 50019 Sesto F.no (Firenze), Italy
¹⁵ Dipartimento di Matematica e Fisica, Università Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
¹⁶ Max Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
¹⁷ California Institute of Technology, MC 249-17, 1200 East California Boulevard, Pasadena, CA 91125, USA
¹⁸ Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, PA 16802, USA
¹⁹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA

Received: 25 July 2017
Accepted: 12 December 2017

Abstract

We imaged, with ALMA and ARGOS/LUCI, the molecular gas and dust and stellar continuum in XID2028, which is an obscured quasi-stellar object (QSO) at z = 1.593, where the presence of a massive outflow in the ionised gas component traced by the [OIII]5007 emission has been resolved up to 10 kpc. This target represents a unique test case to study QSO feedback in action at the peak epoch of AGN-galaxy co-evolution. The QSO was detected in the CO(5 − 4) transition and in the 1.3 mm continuum at ~30 and ~20σ significance, respectively; both emissions are confined in the central (<2 kpc) radius area. Our analysis suggests the presence of a fast rotating molecular disc (v ~ 400 km s⁻¹) on very compact scales well inside the galaxy extent seen in the rest-frame optical light (~10 kpc, as inferred from the LUCI data). Adding available measurements in additional two CO transitions, CO(2 − 1) and CO(3 − 2), we could derive a total gas mass of ~10¹⁰ M_⊙, thanks to a critical assessment of CO excitation and the comparison with the Rayleigh–Jeans continuum estimate. This translates into a very low gas fraction (<5%) and depletion timescales of 40–75 Myr, reinforcing the result of atypical gas consumption conditions in XID2028, possibly because of feedback effects on the host galaxy. Finally, we also detect the presence of high velocity CO gas at ~5σ, which we interpret as a signature of galaxy-scale molecular outflow that is spatially coincident with the ionised gas outflow. XID2028 therefore represents a unique case in which the measurement of total outflowing mass, of ~500–800 M_⊙ yr⁻¹ including the molecular and atomic components in both the ionised and neutral phases, was attempted for a high-z QSO.