We present an ALMA high-resolution ( 0.18 ^ { \prime \prime } \times 0.21 ^ { \prime \prime } ) observation of the 840 \mu m continuum and [ CII ] \lambda 157.74 \mu m line emission in the WISE-SDSS selected hyper-luminous ( WISSH ) quasi-stellar object ( QSO ) J1015 + 0020 , at z \sim 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 \mu m. The host galaxy of J1015 + 0020 shows a star formation rate ( SFR ) of about 100 M _ { \odot } yr ^ { -1 } , while the total SFR of the QSO and its companion galaxies is a factor of \sim 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 } \sim 6 \times 10 ^ { 9 } M _ { \odot } .
As we resolve the [ CII ] emission of the QSO , we can compute a dynamical mass of M _ { dyn } \sim 4 \times 10 ^ { 10 } M _ { \odot } .
This translates into an extreme ratio M _ { dyn } / M _ { BH } \sim 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 10 ^ { 11 } M _ { \odot } at z \sim 4.4 .
These sources will likely merge and develop into a giant galaxy of \sim 1.3 \times 10 ^ { 12 } M _ { \odot } .
Under the assumption of constant \dot { M } _ { acc } or \lambda _ { 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 .