ALMA Observations Show Major Mergers Among the Host Galaxies of
  Fast-growing, High-redshift Supermassive Black Holes
  We present new ALMA band-7 data for a sample of six luminous quasars at
z~4.8, powered by fast-growing supermassive black holes (SMBHs) with rather
uniform properties: the typical accretion rates and black hole masses are
L/L_Edd~0.7 and M_BH~10^9 M_sol. Our sample consists of three "FIR-bright"
sources, which were individually detected in previous Herschel/SPIRE
observations, with star formation rates of SFR>1000 M_sol/yr, and three
"FIR-faint" sources for which Herschel stacking analysis implies a typical SFR
of ~400 M_sol/yr. The dusty interstellar medium in the hosts of all six quasars
is clearly detected in the ALMA data and resolved on scales of ~2 kpc, in both
continuum (\lambda_rest ~150 um) and [C II]157.74 um line emission. The
continuum emission is in good agreement with the expectations from the Herschel
data, confirming the intense SF activity in the quasar hosts. Importantly, we
detect companion sub-millimeter galaxies (SMGs) for three sources -- one
FIR-bright and two FIR-faint, separated by ~14-45 kpc and <450 km/s from the
quasar hosts. The [C II]-based dynamical mass estimates for the interacting
SMGs are within a factor of ~3 of the quasar hosts' masses, while the continuum
emission implies SFR(quasar)~(2-11)xSFR(SMG). Our ALMA data therefore clearly
support the idea that major mergers are important drivers for rapid early SMBH
growth. However, the fact that not all high-SFR quasar hosts are accompanied by
interacting SMGs and the gas kinematics as observed by ALMA suggest that other
processes may be fueling these systems. Our analysis thus demonstrates the
diversity of host galaxy properties and gas accretion mechanisms associated
with early and rapid SMBH growth.
