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The WISSH quasars project
V. ALMA reveals the assembly of a giant galaxy around a z = 4.4 hyper-luminous QSO

M. Bischetti, E. Piconcelli, C. Fergulio, F. Duras, A. Bongiorno, S. Carniani, A. Marconi, C. Pappalardo, R. Schneider, A. Travascio, R. Valiante, G. Vietri, L. Zappacosta, F. Fiore

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 MBH ∼ 6×109 M. As we resolve the [CII] emission of the QSO, we can compute a dynamical mass of Mdyn ∼ 4×1010 M. This translates into an extreme ratio Mdyn/MBH∼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 Ṁacc or λEdd 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.

Keywords
quasars: individual: SDSSJ101549.00+002020.03, galaxies: high-redshift, galaxies: star formation, quasars: supermassive black holes, quasars: emission lines

Notes
Based on data from ALMA cycle 4 program 2016.1.00718.S.

Astronomy and Astrophysics
Volume 617, Article Number A82, Number of pages 12
2018 September

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia