The WISSH quasars project

V. ALMA reveals the assembly of a giant galaxy around a z = 4.4 hyper-luminous QSO^⋆

¹ INAF - Osservatorio Astronomico di Roma, Via Frascati 33, 00078, Monte Porzio Catone (Roma), Italy
e-mail: manuela.bischetti@oa-roma.inaf.it
² Università degli Studi di Roma “Tor Vergata”, Via Orazio Raimondo 18, Roma, 00173, Italy
³ INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143, Trieste, Italy
⁴ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, Roma, 00146, Italy
⁵ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
⁶ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
⁷ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Centro de Astronomia e Astrofísica da Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
⁹ Instituto de Astrofísica e Ciencias do Espaço, Universidade de Lisboa, OAL, Tapada da Ajuda, 1349-018 Lisboa, Portugal
¹⁰ Università degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Roma, Italy
¹¹ Excellence Cluster Universe, Technische Universität München, Boltzmannstr., 85748 Garching, Germany
¹² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany

Received: 17 April 2018
Accepted: 19 May 2018

Abstract

We present an ALMA high-resolution (0.18″ × 0.21″) observation of the 840 μm continuum and [CII] λ157.74 μm line emission in the WISE-SDSS selected hyper-luminous (WISSH) quasi-stellar object (QSO) J1015+0020, at z ∼ 4.4. Our analysis reveals an exceptional overdensity of [CII]-emitting companions with a very small (<150 km s⁻¹) velocity shift with respect to the QSO redshift. We report the discovery of the closest companion observed so far in submillimetre observations of high-z QSOs. It is only 2.2 kpc distant and merging with J1015+0020, while two other [CII] emitters are found at 8 and 17 kpc. Two strong continuum emitters are also detected at <3.5 arcsec from the QSO. They are likely associated with the same overdense structure of J1015+0020, as they exceed by a factor of 100 the number of expected sources, considering the log(N)–log(S) at 850 μm. The host galaxy of J1015+0020 shows a star formation rate (SFR) of about 100 M_⊙ yr⁻¹, while the total SFR of the QSO and its companion galaxies is a factor of ∼10 higher, indicating that substantial stellar mass assembly at early epochs may have taken place in the QSO satellites. For J1015+0020 we computed a black hole mass M_BH ∼ 6 × 10⁹ M_⊙. As we resolve the [CII] emission of the QSO, we can compute a dynamical mass of M_dyn ∼ 4 × 10¹⁰ M_⊙. This translates into an extreme ratio M_dyn/M_BH ∼ 7, i.e. two orders of magnitude smaller than what is typically observed in local galaxies. The total stellar mass of the QSO host galaxy plus the [CII] emitters in the ALMA field of view already exceeds 10¹¹ M_⊙ at z ∼ 4.4. These sources will likely merge and develop into a giant galaxy of ∼1.3 × 10¹² M_⊙. Under the assumption of constant Ṁ_acc or λ_Edd equal to the observed values, we find that the growth timescale of the host galaxy of J1015+0020 is comparable or even shorter than that inferred for the SMBH.