The WISSH quasars project

IX. Cold gas content and environment of luminous QSOs at z ∼ 2.4–4.7

¹ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
e-mail: manuela.bischetti@inaf.it
² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
³ Dipartimento di Matematica e Fisica, Universitá Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁴ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de las Astronomía, s/n, 18008 Granada, Spain
⁵ Departamento de Física Teorica, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain
⁶ European Southern Observatory (ESO), Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Magrans, 08193 Barcelona, Spain
⁸ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Avda Vicuna Mackenna 4860, 8970117 Macul, Santiago, Chile
⁹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁰ INAF – Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, 00133 Roma, Italy
¹¹ Dpto. de Ciencias Fisicas, Universidad Andres Bello, Campus La Casona, Fernandez Concha 700, 7500912 Santiago, Chile
¹² Dipartimento di Fisica e Astronomia, Alma Mater Studiorum Universitá di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁴ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁵ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 2, 50125 Firenze, Italy
¹⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁷ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
¹⁸ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
¹⁹ INAF – Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²⁰ Instituto de Astrofísica e Ciências do Espa, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
²¹ Centro de Astrobiología (CAB, CSIC-INTA), Dpto. de Astrofísica, Ctra de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
²² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²³ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, Via A. Corti 12, 20133 Milano, Italy

Received: 29 July 2020
Accepted: 10 October 2020

Abstract

Context. Sources at the brightest end of the quasi-stellar object (QSO) luminosity function, during the peak epoch in the history of star formation and black hole accretion (z ∼ 2−4, often referred to as “Cosmic noon”) are privileged sites to study the cycle of feeding & feedback processes in massive galaxies.

Aims. We aim to perform the first systematic study of cold gas properties in the most luminous QSOs, by characterising their host-galaxies and environment. These targets exhibit indeed widespread evidence of outflows at nuclear and galactic scales.

Methods. We analyse ALMA, NOEMA and JVLA observations of the far-infrared continuum, CO and [CII] emission lines in eight QSOs (bolometric luminosity L_Bol ≳ 3 × 10⁴⁷ erg s⁻¹) from the WISE-SDSS selected hyper-luminous (WISSH) QSOs sample at z ∼ 2.4−4.7.

Results. We report a 100% emission line detection rate and a 80% detection rate in continuum emission, and we find CO emission to be consistent with the steepest CO ladders observed so far. Sub-millimetre data reveal presence of (one or more) bright companion galaxies around ∼80% of WISSH QSOs, at projected distances of ∼6−130 kpc. We observe a variety of sizes for the molecular gas reservoirs (∼1.7−10 kpc), mostly associated with rotating disks with disturbed kinematics. WISSH QSOs typically show lower CO luminosity and higher star formation efficiency than infrared matched, z ∼ 0−3 main-sequence galaxies, implying that, given the observed SFR ∼170−1100 M_⊙ yr⁻¹, molecular gas is converted into stars in ≲50 Myr. Most targets show extreme dynamical to black-hole mass ratios M_dyn/M_BH ∼ 3−10, two orders of magnitude smaller than local relations. The molecular gas fraction in the host-galaxies of WISSH is lower by a factor of ∼10−100 than in star forming galaxies with similar M_*.

Conclusions. Our analysis reveals that hyper-luminous QSOs at Cosmic noon undergo an intense growth phase of both the central super-massive black hole and of the host-galaxy. These systems pinpoint the high-density sites where giant galaxies assemble, where we show that mergers play a major role in the build-up of the final host-galaxy mass. We suggest that the observed low molecular gas fraction and short depletion timescale are due to AGN feedback, whose presence is indicated by fast AGN-driven ionised outflows in all our targets.