Abstract
The outstanding mass growth of supermassive black holes (SMBHs) at the
Reionisation Epoch and how it is related to the concurrent growth of their host
galaxies, poses challenges to theoretical models aimed at explaining how these
systems formed in short timescales (<1 Gyr). To trace the average evolutionary
paths of quasi-stellar objects (QSOs) and their host galaxies in the BH
mass-host mass ($M_dyn$) plane, we compare the star formation rate (SFR),
derived from the accurate estimate of the dust temperature and the dust mass
($T_dust, M_dust$), with the BH accretion rate. To this aim, we
analysed a deep, $900$ pc resolution ALMA observation of the sub-mm continuum,
CII and H$_2$O of the $z6$ QSO J2310+1855, enabling a detailed study of
dust properties and cold gas kinematics. We performed an accurate SED analysis
obtaining a dust temperature of $T_dust = 71$ K and a dust mass of
$M_dust= 4.4 10^8\ M_ødot$. The implied AGN-corrected SFR
is $1240 \ M_ødotyr^-1$, a factor of 2 smaller than previously
reported for this QSO. We derived the best estimate of the dynamical mass
$M_dyn = 5.210^10\ M_ødot$ within $r = 1.7$ kpc, based on
a dynamical model of the system. We found that $SFR/M_dyn>\dot
M_BH/M_BH$, suggesting that AGN feedback might be efficiently
acting to slow down the SMBH accretion, while the stellar mass assembly is
still vigorously taking place in the host galaxy. In addition, we were also
able to detect high-velocity emission on the red and blue sides of the CII
emission line, that traces a gaseous outflow, and for the first time, we mapped
a spatially-resolved water vapour disk through the H$_2$O v=0
$3_(2,2)-3_(1,3)$ emission line detected at $\nu_obs = 274.074$ GHz,
whose kinematic properties and size are broadly consistent with those of the
CII disk.
Description
The black hole and host galaxy growth in an isolated $z\sim 6$ QSO observed with ALMA
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