Kiloparsec-scale ALMA Imaging of [CII] and Dust Continuum Emission of 27
  Quasar Host Galaxies at z~6
  We present a study of the [CII] 158micron line and underlying far-infrared
(FIR) continuum emission of 27 quasar host galaxies at z~6, traced by the
Atacama Large Millimeter/submillimeter Array at a spatial resolution of ~1
physical kpc. The [CII] emission in the bright, central regions of the quasars
have sizes of 1.0-4.8kpc. The dust continuum emission is typically more compact
than [CII]. We find that 13/27 quasars (approximately one-half) have companion
galaxies in the field, at projected separations of 3-90kpc. The position of
dust emission and the Gaia-corrected positions of the central accreting black
holes are cospatial (typical offsets <0.1"). This suggests that the central
black holes are located at the bottom of the gravitational wells of the dark
matter halos in which the z>6 quasar hosts reside. Some outliers with offsets
of ~500pc can be linked to disturbed morphologies, most likely due to ongoing
or recent mergers. We find no correlation between the central brightness of the
FIR emission and the bolometric luminosity of the accreting black hole. The
FIR-derived star-formation rate densities (SFRDs) in the host galaxies peak at
the galaxies' centers, at typical values between 100 and 1000 M_sun/yr/kpc^2.
These values are below the Eddington limit for star formation, but similar to
those found in local ultraluminous infrared galaxies. The SFRDs drop toward
larger radii by an order of magnitude. Likewise, the [CII]/FIR luminosity
ratios of the quasar hosts are lowest in their centers (few x10^-4) and
increase by a factor of a few toward the galaxies' outskirts, consistent with
resolved studies of lower-redshift sources.
