ALMA 200 pc imaging of a z~7 quasar reveals a compact, disk-like host
  galaxy
  We present 0".035 resolution (~200 pc) imaging of the 158 um [CII] line and
the underlying dust continuum of the z=6.9 quasar J234833.34-305410.0. The 18 h
ALMA observations reveal extremely compact emission (diameter ~1 kpc) that is
consistent with a simple, almost face-on, rotation-supported disk with a
significant velocity dispersion of ~160 km/s. The gas mass in just the central
200 pc is ~4x10^9 M_sun, about a factor two higher than that of the central
supermassive black hole. Consequently we do not resolve the black hole's sphere
of influence, and find no kinematic signature of the central supermassive black
hole. Kinematic modeling of the [CII] line shows that the dynamical mass at
large radii is consistent with the gas mass, leaving little room for a
significant mass contribution by stars and/or dark matter. The Toomre-Q
parameter is less than unity throughout the disk, and thus is conducive to star
formation, consistent with the high infrared luminosity of the system. The dust
in the central region is optically thick, at a temperature >132 K. Using
standard scaling relations of dust heating by star formation, this implies an
unprecedented high star formation rate density of >10^4 M_sun / yr / kpc^2.
Such a high number can still be explained with the Eddington limit for star
formation under certain assumptions, but could also imply that the central
supermassive black hole contributes to the heating of the dust in the central
110 pc.
